Hurray, progress. This morning I checked MyDrive Connect and there was a new community mapshare waiting to be updated. It was bigger than the previous ones that had failed (6kb vs usually 3kb) and when I tried it the updating proceeded slowly but perfectly ok. None of the previous issues of stalling at 1 minute 47 seconds. So I'm presuming that TomTom have finally found a resolution to this problem.
EDIT: No, it was too good to be true. After reporting that it has successfully installed the community share map update, I disconnected the TomTom from the computer. I've just reconnected it to find there is 'another?' csm update ... so I tried it and guess what, it froze at 1 minute 47 seconds. I give up.