Many thanks to everyone who responded to my question, especially Ruth,
I particularly like her idea of turning the work by hand to see what's
happening at the tool (and I like her web site).
I managed to sort the problem by using a scraper but it's quite
worrying how quickly a catch can happen. I did the inside of the bowl
easily (much to my surprise!) but just coming on to the rim nearly
wrecked it.
Thanks again
Bill