Oh, your in the UK? I have a bad habbit of just thinking everyone is from the US unless I see it stated elswise. Geuss that's just becuase about 95% of the people I have forum contacts with actually are.
Anyway,
"If you want your batteries to last for the short term, any charger will do. If you want your batteries to last the maximum possible lifespan, then you need a sophisticated charger that charges in a three (or four) stage sequence, bulk charge (fast and efficient), finish charge and float. If you don't have that sophisticated charge managment system, good batteries with a potential ten year life will only last 3 or 4."
I think that's kinda what the Vector and other Smart Chargers are. Supposedly monitor the battery and adjust the charging rate by how the battery excepts the charge or something like that. My Vector will start at 40amp on the digital display when I set it for 40amp, but then a fairly dead battery will go to about 42 amps charging for awhile, in a bit it drops to 39amps and you can sort of watch it reduce the amps. Come back in a few minutes and it shows 35 amps, an hour latter maybe 23 amps ect. Also sometimes shuts down to let the battery cool or adjust I geuss. Since it does it often after just a few seconds or minutes after starting a charge I know it not to let the charger itself rest.
Mine was the biggest (heavy duty one) I've seen sold, was about $100 I think when I bought mine, now I think about $80. It's a Vector Smart Charger with a desulfate mode (pulse chager) and 2/10/20/40 amp settings with 100amp boost for engine starting. I have seen smaller cheaper ones going up to only 10 or 20 amps.
I got mine at Wal-mart in the US, but I am sure you could probably get basically the same thing in the UK somewhere too.
As far as that green dot on the batteries they are worthless it seems, I also have batteries I bought new those things don't work on now, batteries are fine though. I never wanted the dot anyway, it was just there when I bought the battery so I was stuck with it 
If your planning to run a generator to charge your batteries I think I would build one just for that. Hey wait a minute, I have built one
Basically just get a good gas engine from a lawn mower (I used a riding mower of 12hp) and connect some cheap car altenators to it. You can get real fancy or just stick to the basics. Some push mowers use the blade wieght as part of the flywheel sort of, they can be made to work also but you'll need to use a heavy pully on those most likely.
For my basic one, I just took the mower deck off the riding mower, made brackets to fasten the altenator where the mower deck used to be, used the mowers belt (it was the correct size, but not always) I used the GM altenators (often called one wire).
Simply start the mower as normal, hit a hot wire to the altenator for a second to charge it, and adjust the engine speed to produce the most amps. I did have an amp meter installed for that purpose. Basically with the 12hp engine I just used a fast idle and I think I got around 40-60amps with the altenator I was using. The engine ran for hours on one tank of gasoline, not sure the tank size, I "think" about 1/2 gallon.
Since I was only charging one battery at a time back then it worked perfect for me, I had no other power except charging while driving with my vans and a 2 battery setup. It should work well for any size battery bank, you'll get constant amps at a constant speed till the bank starts getting full, just like running a car and charging the battery in it of course.
If you wanted to get fancy with it, you could even install more than one altenator, get a couple 80-100amp ones and charge a couple battery sets at once. The only limit would be the size of your engine. More HP more amps you could get.
The one I am building now will have several altenators, I engage the ones I want by pulling a lever to tighten the belt for them. So I can charge one to 3 set of batteries at the same time just by engaging the number of altenators I want running.
Each will have it's own amp meter. More I engage the more fuel I will burn but the more amps I get. 60/120/180 ect since each will produce at least 60amps. Then of course the built in regulator will reduce that according to the batteries state of charge, but when they are low I will have that much available in the same time period.
As far as chargers, any fast charger should work well for you. The fancy ones may be the best for other uses where power is available fulltime. But float and maintain will do you no good if your using a gennie to power the charger and the genie is not running! No power = no float! And if your burning gasoline propane or other costly feuls your best bet is to just get it done fast and shut down the genny.
I realy would suggest for battery charging you build something like I described with the lawnmower engines if you can do it yourself or have free help that can. If your running a gennie on costly feul, making 110v, running a 40amp charger to make 12v for the batteries you have a great deal of losses and will burn far too much feul in the long run compared to just making 60amps of 12v directly. To a point you may use more fuel for more amps when getting into the larger numbers, but just idling and doing nothing uses alot of feul also. When you look at it that way then adding just a little more feul for alot more amps and a far shorter runtime you actually come out better in the long run. I don't know off hand how hard your gennie would have to work to make the 40amps, if it's barely loaded and your only using half it's available power your basically waisting the other part of the feul. Kinda like Idling a car engine, your charging the battery but wiasting a ton of feul doing it by not using the rest of the hp available in the feul being burned just to keep it running.
Either way to produce 40amps of 12v dc will require the same amount of feul. To produce 110v AC run a charger to convert that to 40amps 12vDC will take MORE feul than just making 40 amps 12vdc directly.
I have somewhat done some testing on the theory of running a large engine with no load and a battery charging load. Having some cars I collect and often running the engines just to keep them working I have used exactly 1 gallon of gas, started an engine and let it run till all gas was used and that was no extra loads. Then I started the engine and connected a low battery bank to the altenator and run on exactly one gallon of gas till it ran out again. With the load applied the engine rpms were lower, you could hear the engine slow down when I connected the load, but the run time was close enough for practical purposes to call it the same. Maybe a minute or 2 less runtime total but being as I got nearly fully charged batteries from it I had used almost no extra feul to do so. That is another way I plan to keep my batteries charged at times. It is not advisable to run a car engine just to charge batteries off the altenator because of the feul used, however I will be having to start my collectables ocasionally and let them run just to keep them working, so I will set a pattern of starting each at a certian time and letting it run on 1 gallon of gas maybe once per month (I will probaly use propane or wood gas actaully) and while the engine is running to keep the seals and gaskets lubed and prevent sticky valves ect.. I will charge batteries at the same time, get a duel use from the feul being burned since I need to run those egines just to keep them working properly anyway. And exhaust heat is useful also with a good heat exchanger
