Bosch ytx14-bs 100a - It maybe cheap because I suspect that 100a is 100cca (Cold cranking amps), you probably want more like 200cca like a Yuasa YTX14-BS or it may struggle to turn the engine over when its really cold.
You can buy the Yuasa £40 delivered from eBay - Midland Batteries I think. I've just bought one for an Aprilia. Avoid Varta - the last one I bought lasted little more than 2 years.