Cheers lads.
it seems the battery size should be fine for the car then. The battery is about 8 months old, a Bosch silver one.
I know the guy I bought it off had the battery fitted thinking it was the problem causing power loss. It clearly wasn't.
He had it on trickle charge all the time when it was on the garage so its never been deeply discharged as far as I'm aware, as when it has gone flat on me its only in terms of not starting the car and Ive charged it straight away.

Going to test voltage drop on alternator wiring in the morning, and might stick a new alternator on it if in doubt.
will update!