I have had trouble with the universal joints of my Morris 1100, both pairs! At 17,000 miles bits of rubber inner universal joint deposited themselves on the road. This caused terrible transmission snatch, as one would expect. As you correctly say, the reason for failure is oil seepage past the differential casing oil seals. I was told that the replacement ones were of a tougher rubber. I suspect that the original joints are the same as the Mini’s and cannot withstand the extra torque of the larger engine. I think this type of failure is not as rare as you suspect.
However, their failure was only the start of the troubles I experienced with the car; the outer universal joints failed, as did a rear hydrolastic unit, a bearing went on the gearbox mainshaft, the discs of the brakes became badly scored, and the remote control tunnel to the gearlever became detached. After all these calamities, together with very heavy oil consumption, I lost faith in the car and sold it. I have heard of other people having trouble with early 1100s, mine being early 1963.
I feel that B.M.C. can only be doing themselves harm by selling cars obviously needing more development before production commences. It would appear that early 1800s are being as troublesome as the early 1100s. I would be most reluctant to buy another 1100, although I concede that the car is, on the whole, a good design.
Redditch. A.J. Venables.