Ok so I have a brand new alternator and a brand new cable from there to the starter. It had got so hot that it had melted/burned off several cm of insulation! I also have a brand new 900cca battery, and the car is starting and running much better. Still not quite as lively as others ive heard so probably a starter motor replacement on the cards sometime in the future too.
Anyway, despite all that, still hearing zaps from the alternator area! Has been suggested possibly static from the belt? Have checked all earths (motor came out to swap the alternator)
Any ideas? Next plan is to try and locate the exact source of arcing, need to get on a hoist in the dark