Question

How can I find out which fuse is for what on caravan I just bought, Can get lights on but not plug sockets to work

Answer

The lights in a touring caravan generally operate from the 12 volt battery system, as does the water pump, the cassette toilet and any 2 pin sockets that you may have. These are all fused at the fuse board.
The three pin sockets are 230 volt mains and of course will only operate when you are connected to a mains supply via hook-up cable. These sockets are controlled by a consumer unit with the RCD and MCB safety devices. All switches on the consumer unit must be in the 'up' position for the 3 pin sockets to work.