Thanks Everett - that's exactly what I intend to do.

We will be testing two new sitemaps with 100 x URLs each. 1. With just the file extension removed and 2. With the entire cropping part of the URL removed, as suggested by Matt.

Will be interested to see whether just one or both of the sitemaps are successful.  Will of course post the outcome here, for anyone who might have this problem in future.