ok so here's the first attempt at a repair....
Make a shim: take an aluminium tube (from and extending hose brush) , OD 22mm, ID 20mm, cut off 7mm length and epoxy it into the recess.
Obtain a smaller oil seal, add a bit of hylomar to the outside of seal and fit.
It's not leaking anymore. will see how it goes. Total cost to me so far... zero plus oil seal. I suspect if it fails I will get a shim machined to fit properly.
Thanks to Bob Wright for the idea.