Might sound daft BUT take your multi meter with you on a ride..when it wont start check its still holding the voltage you recorded after charge.

Tbh Id put money on it being the battery. It may hold the voltage BUT may not have enough amps to turn the motor.

Buy the best battery you can afford. I bought one from Halfords,YUASA YB9B-BS. �57..less if you have a trade card.