Jim,
Indeed around 100uF to 150uF. Last night we had the opportunity to try again and started by connecting a 80uF start cap from another motor, that was a significant improvement on the previous 40uF or 320uF, but not totally OK. We continued adding some more fluorescent caps up to about 130uF, which is all we had, and that seemed to be a major improvement and worthy of trial back in the mill. I operated the start cap contacter manually while by mate operated the mains switch and loaded the flat belt pulley with some timber and 130uF looks very promising. Next step is to put the motor back in the mill and connect the new belt drive and purchase a suitably sized start cap and try again. He has enough fluorescent caps that we can do some more tuning when fitted in the mill to try the real starting performance when driving the intended load. From some initial checks the 40uF run capacitance seems OK from the voltages mentioned by Richard, we'll check again under working conditions. Will report back again with hopeful success and a working Hayes Diemaster mill and original, if slightly altered, Brooks 3 phase motor. Maybe my mate will respond when he figures out newsgroups, I think this is the first he has looked at.
Many thanks to all for your information.
Dave