Have had a couple more goes, uninstalling and reinstalling the program in between (which seems to be the only way to get it to do anything at all).
First time I avoided a few files from an odd source that I thought might be a problem: got to 1083.
Second time avoided these and a few more related files: got to 1000 still stuck in the middle of the same folder, so thought there might be some sort of limit at or around 1000 (nice round number) so ...
Third time tried importing each month separately: got 5 months OK (this included the one where I'd got stuck halfway through before) = 1063 files, tried June and ... nothing. Sitting there telling me it's 'importing files ...', but with 0 imported.
There is no Adobe support for trial versions, so ... not a great confidence builder if they are expecting me to dish out $186 on the off-chance that the paid version might work!