Thanks for your help guys. Similar answers from both sides, seems to be a choice of either a car adapter or a bare board type converter. The car type is likely to be easier, in fact I think it would be a good idea to fit a couple of 'cigar lighter' type sockets when I do a refit in the winter.
Regarding the two battery setup, I will keep them as engine battery and leisure battery, but probably fit a split charging circuit so that the leisure battery is topped up by the engine once the engine battery is full in case the solar panel doesn't supply enough once I add more lights, laptop, audio, shower pump...
On the other hand it would be good to have the solar panel keep the engine battery topped up when we don't use it for a while, so will investigate split charging in both directions. Might get complicated unless someone makes a specialist control box for just that scenario, in which case it'll get expensive!
-- Phil Thane
www.pthane.co.uk phil@pthane.co.uk 01767 449759 07582 750607 Twitter @pthane