Generally acknowledged to be excellent is the Elecsol with a 5 year warranty at £85
However, most leisure batteries will serve your puropse adequately. You can get an 85AH leisure battery with a 3 year warranty at most reputable motor accessory shops for about £60.
we had some difficulty getting a leisure battery to fit the unit we have. It has an outside compartment. So we trawled quite a few places and eventually ended up at the local agricultural shop. they keep them for small electric fences. they have from 40 - 110 Ah. cost about £40 to £65. If you are only going to use it for lights and water pump the battery will last for ages before needing charging. Means you can do without EHU for days.
I'm not sure which socket you mean, but for minimal power for the pump, any old car battery which has some life in it will do.
The big leisure batteries are really only required if you run loads of things off them for about a fortnight.
So if you have, or know someone who has one lying about their garage, try that, because that doesn't bother the bank at all. I used one in my caravan for several months (not all in one go!!), which powered all sorts of things.
Alternatively, a little car battery might well do at about £40, somewhat along the lines suggested by Esermalda
Most leisure battery die at an early age, you could use an ordinary car battery much cheaper plus many have a breather pipe that you can run a length of windscreen washer pipe out through a small in the locker floor. I personaly use a very old but good power supply that is 17amp that I run all my 12v from, it is hard wied into my 240v system
------------- If it ain't broke don't try to fix it!
I got one from a local motor factoirs for about £30. Don't buy a car battery, they are made in a different way as the load on a car is completely different to a tent.