I am milling - in metric
I am starting at X0 Y0

I want to move out to X20 (which is the side of the workpiece) then mill up to Y20
The simple code is G0 X20, G1 Y20 - but in reality I want the table to move to position X24 and I only want to mill up to Y16 to take account of the tool diameter (8mm) (I then want to move back to Y0, move in 0.5mm to X19.5 and take another cut, and so on, down to 10mm)

I have tried several combinations of G40, 41 and 42, but I do not seem to have struck the right one. I can get the tool to compensate for diameter, but never in the direction, or on the axis that I think - HELP

You need a lead in move for the comp to be applied. Something like this:

G40 (make sure comp is off)
G0 X-5 Y-5
G42 R4 (turn on comp right and call out tool radius)
G0 X0 Y0
G0 X20
G1 Y16
G40 (turn off comp)

Then repeat as needed.

Thanks - I'll try along those lines and see if I can crack it.