I'm limited to what size of blade I can get on the turbine by my location.  I'm in a very built up area with neighbours on all sides and I'm flying what I feel is safe for the site.  If I was out in the sticks with nothing around me then I'd certainly be going bigger.  I also felt that a good blade should out-perform a bad blade of the same size.

@Flux,

I know you've mentioned going to 24v before, but it would require an extensive change of everything I have set up. I will have a try with just the batteries and see what happens.  
I have tried disconnecting the mill from any load and yes it does spin up, but I've never let it get too fast, but do see a kick on the ammeter when I reconnect.

I don't think I have any shorts or bad diodes in the rectifier, it's a 6 stud set-up and seems to check out ok.  I've had a 'scope on the line from the mill (across the line resistance gives enough to see a trace) and I'm not seeing any gaps, just a continuous nnnnnnnnnn.  I have noticed that just before cut-in I do see a couple of humps on the base as some diodes start to work before others.

I could just be that there is a lot less wind than I think, the new leaves on the trees tend to exagerate what we do have, and for some time now it's been coming from the wrong direction to be really useful in my garden.

Thanks for the replies, if I've missed anything, I'll go back and reply to individual comments.