i would use a lorry battery or a leisure battery
110 amp hour lorry batteries are really cheap compared to car batteries. £60 for a 3 year warranty 110ah.
the rationale behind this is that you have plenty of spare capacity, and SLA(start lighting and auxiliary) batteries are ruined by deep discharge, by using a 110 ah you only use about 10% of the capacity even if you leave gps and radio on for a few hours.
Leisure batteries are great(for deep discharge) but you need to make sure they have a cranking capacity, most are designed for engine starting some arent. the cranking capacity will be lower than a SLA and they are more expensive.
I went for a 663 (or 664 terminals other way round)on mine (big lorry battery) cos they are the cheapest option and suitably big!
buy from an auto electrical factors they are cheaper than car batteries(being very common fitment)
I doubt you will get a 35ah car battery in halfrauds for the price of a 663.
theyre on Ebay for £59+postage
Before you buy a battery put a voltmeter across the battery, when you try and crank the engine, if the battery is cream crackerd the voltage will drop.
if it stays around 11-13 volts then you have corroded cables and connectors.