Martyn,
The size of battery you need depends largely on the use you put it too. If you only go on EHU sites you only need a small battery to help regulate the 12v current from the transformer (some even risk it and go without). If you use non EHU sites but with little extra 12v equipment and only for a weekend an 85Ah is probably quite adequate. If you use a lot of 12v equipment or stay away for more than just the weekend I would go for the largest battery you can fit in your battery box (our 120Ah battery will last 7-10 days but no TV etc.) after all there's not that much differencein the price!
Alan