You should be able to pick up a decent 12v leisure/marine battery (which is designed for repeated discharge/recharge cycles and a continuous drain), with a years warranty, for about £60-£70 from your local camping/caravan store or GoOutdoors. You'd need to collect as shipping costs would be huge due to the weight!
Car Batteries are designed for short, high discharge conditions, such as turning over a starter motor etc and not best suited for 'our' uses (although they will work but won't give as long service as a leisure battery).
HTH
