Can anyone help with the following - I have made a new plug 10mm long plug to fit inside the oil pump distributor shaft and I assume this goes into the internal bore and sits on the machined lip down inside the distributor shaft just before the oil pump plunger - is this correct?

Syd you old sausage . Surely a wee bit of maths would give you the answer you want .
DEET has given you the length of the plug . I supplied the depth measurement privately .
If the two measurements added together match the depth of the internal lip , 
then you have your answer

Snoop56 me old currant bun, good to see you on your keyboard on this cold chilly Christchurch morning. Just want to be 100% sure that the plug sits against the machined lip as after its epoxied in place it would be a job to remove.

The two measurements don't match the internal depth from the top of the shaft to the lip, however a couple of mm to spare it seems it will not be a problem.