Thanks for the replies.
I'm based in London, England. The panel I have should be able to thermosyphon, as the company that makes it makes a version popular in China that does exactly that. The difference is that instead of the tubes going into a manifold, they go directly into a tank built onto the top of the panel, as you can see in the version here:
and there's a better diagram of how this type of system works here
In this style of panel the water naturally thermosyphons down the side of the tubes and back up into the main tank. My plan is to have a tank above the panel where the in and out from the manifold connect to the tank. I figured I might have to experiment with putting the panel on an angle to give gravity a bit of a hand.
Alternatively I could find an old tank and see if I could use the fittings from the manifold and fit them directly into the wall of the tank, but that would probably lead to leakage unless it's done really well.
I didn't want to fit a tank inside the house as I'm having to build down to a low weight and I'm very tight on space and budget. I did consider a tank in a corner but I'd have to drain the system each time I move and I'm going to be using rainwater for my water supply, not connected to the mains. Also, that would mean putting the panel on the roof of the cabin, and I'm not sure if the panel would survive being driven down the highway unless I take the tubes out each time I move.
Now, I'm not intending to move often, it's not an RV, more of a moveable home. If I went for a regular type of system, I guess I'd need a 130 liter or so insulated tank (is there a way to figure out the minimum?) and a 12v circulation pump. Does the pump have to be on constantly or just occasionally. My power supply for the cabin will be a 12v 220ah battery pack charged from two 85w pv panels so I'd need to keep things as simple and low power as possible.
Ron, thank you for the link to that book, I shall be reading my way through it over the next few days.