Thanks Geoff, I remember reading / seeing that post now as well as reading about using a camera to check gearbox too.
I think I was concerned as I couldn't see any sprocket damage so thought I needed to find it. Perhaps the very first thing I should have done was contact previous owners and asked for bike history.
Still needed to strip it down thou, inspect, hone, rings, clean and paint. Just cost a bit, cheap if you consider conscience.
The bike; I've bought another power tool

I bought a cheap (£50) polisher / sander / grinder to cut back the poor lacquer job on the tank, if you want to save a lot of money give it to a painter, but I'm glad I've gone this route, the polisher is doing the job but I petrified I'm gonna rip though the lacquer to the base coat or flip the tank onto the floor. photos later.
Tank decal fitting; I'm guessing look at a load of tank photos I've been collecting and have a go? or is there an easier / more helpful way? Please can someone give me a rough distance from the red blocks to the black bars? As I feel get this wrong it'll look pants. Oh and I'll try to get them on the right way and if I don't, well I don't think it matters does it.