Brief update for you all.

I had a small gremlin in the drive system, in that when freewheeling the chain to the back wheel would sometimes throw itself off, this was because the sprocket was from a freewheel with those helpful notches to allow it to change easily.

Anyway got a better sprocket and after some filing got it to fit.

Despite it being dark here I fitted some lights and took it for a fast test blast and despite plenty of fast freewheeling the chain never came off - success yippeeeeeeeee.