Finally, it is is "finished" (I can now see improvements so I guess it never will be!). Originally, I was going to use timing belt and cutting the timing pulleys was a bit of a stumbling block, not having a dividing head or similar. Poly-V (multi-rib) belt made that problem go away.
This is the new bottom end pulley to drive the countershaft and increase the top end speed:
This is the finished mod in "low gear":
The countershaft pulley is 130mm x 20mm in cast iron, I think ally might have been a better choice, its quite a flywheel.
I cheated by buying a C6 "banjo" and counter shaft spindle from Ketan at Arcurotrade. The mod would have been cheaper if I'd fabricated the thing from barstock, but I can't weld (yet) and don't have the gear to braze that mass of steel.
The pulleys are home made.
The ratios available are roughly motor:spindle 8:1 and 1.7:1 instead of the original 2:1.
It still won't take quarter inch cuts at 50revs, but it does have real grunt now, so much so that I'm tempted to leave the drive belt a bit slack so I don't get into serious trouble, particularly when parting off.
This picture shows the CD I made with 60 holes around the edge to drive a rev counter:
This is what the stupid questions have been about over the past months - thanks guys!
Steve