John, have you checked the earth strap from the battery down onto the gearbox? Amazing how things can come loose after a major rebuild. This one is only really needed for the starter and charging current as all the rest of the earthig is on the bodyshell and things like stop solenoids have their own earths and can tolerate bad ones as the currents involved are tiny.
Either that or simply bad battery connections. Don't rule out a duff battery as well. They can seem fine until they are called on to deliver a couple of hunderd amps to the starter and then object. It only takes one cell to be a little off-colour and the whole battery is duff.
Did you do up the nut that secures the heavy starter feed cable onto the solenoid. I know this nut of old
Access is not the best and it can feel done up when it isn't.
My massive sorrys if this all sounds a bit granny suck eggs to one as able as you John...
Starter motors either stop totally or start to work intermittently. It is rare indeed for a starter to suddenly run very slowly. A duff solenoid normally goes intermittent at first. My bet is the starter itself is OK.
If you try a start with headlamps on, do they dim appreciably when trying to start? If they stay nice and bright, the battery is OK. have you tried jumping it?
I've known them die competely that suddenly John but never like that, good enough to keep headlamps bright with a very slow starter. 
