My G is a 1980 280 GE. It came equipped with the same removable U-joints with circlips that you describe, Chuko. One old U-joint I took out is here on the bench and it measures 30mm cup diameter and 82mm across the cups.
The spare parts I have (of which one has already been used to replace the joint mentioned above) are GWB brand parts and they carry the part number:
7 287 10 06 00 400
I had Drive Train Industries in Denver Colorado order these for me, but they had to buy them from another source. One of the two boxes has the Greenville, South Carolina address that Harald mentioned. The label on the other box is from a company called "Machine Service Industrial" in Racine, Wisconsin, USA. I think this is their website:
http://www.machineservice.com/welcome.htm
Without knowing more about where you're located it's tough to know whether this info is of any value. But in any case, armed with the part numbers from GWB and GKN, hopefully you can obtain the right parts from a local dstributor for one of those companies. Failing that, maybe knowing the size will allow you to source a suitable substitute.
Good luck!
-Dave G.