@creaseuk, did this last year on a LIKE 200i.
Took bearings and seats out, went to a PARKER Industrial store and searched together with the sales guy. Found a couple of roller bearings which fitted from OD and ID but were a little bit to wide/high.
So I had to lathe on the middle stem, to make them fit. It's possible but requires IMO a well equipped workshop. Getting out the seatings from framepart is a challenge too.
My opinion, take normal sphere bearings, some really tough / sticky grease and "glue" the spheres in to mount. Way easier than to go through this other hassle.