Thanks, todays blunders then:-
Estimated the belt size based on the pulley I have, bought the belt, bored the pulley, cut the keyway. Checked the motor speed 2800.... bugger...
I think, since my top pulley is smaller than original, I need a smaller motor pulley, as well as a 1400 motor. For now I'll carry on with it, since for low power stuff the vsd will cope with it.
Thanks all for the help.
I'm a fan of taper lock pulleys/bushes here. Lets you swap about very easily particularly if you have a lot of motor adjustment available (or a wide selection of belts!)