I don't know whether or not you have resolved this problem but there are currently 56 results showing for the enquiry 'electric window motor' on eBay once it is narrowed down to 'Rover' only. Some of these include the frames/regulators and some are advertised as new. You may be able to find one there by comparison with what you have and then check the dimensions by emailing a question to the seller. Please update us with information and progress, as I'm sure it will be a problem which will face a few of us in the years to come. Good luck.

How do they know it was a Big Bang if there was no one around to hear it?