Dont buy a battery from Halfords if you can help it.
I take it you are buying a Leisure battery and not a car battery. They are subtle differences and its all down to the charging and the retention of the charge. Leisure batteries are normally more expensive.
I bought a 110Ah battery for the van when I put the mover on the 80Ah would just about cope withe the van's needs but not the mover and the van. I did not toss away the 80Ah just hooked it up in parallel so I have nearly 190Ah capacity on the van.
I got mine off ebay about ?60 plus postage(it was covered with stamps
)but definitely less than ?80
Try Towsure,or maybe Burnhouse Caravans down the A737 they seemed reasonable for caravan gear
One other possibility is Dingbro, the are supposed to be trade only but I am sure you can brassneck it. You can get a new battery for ?30 for a Peugot 206, Halfords are a tad more than that
Hope it helps,
Tommy