John, when I had my gearbox issues (which Alan kindly helped with) I looked at prices etc. and there was 5 speeds from the wreckers for much cheapness. Rebuilds looked like a complete lottery in terms of prices. Thankfully mine got sorted, although I'm yet to be convinced that some damage wasn't done to curtail its life.
I know it looks like it is the box now - but worth checking the hubs/bearings? At knockers you're putting an awful lot of load on the passenger side ones and I know I'm not the only one that's had issues with them very soon after a track day. Admittedly not a complete failure like that but worth a check.