As the spare unit was 25 miles away then the day could not be redeemed and we removed the offending item at the end of the day. Bits of molten copper dropping out of it convinced me it was time to send it to the repair shop!
I've since fitted the replacement 'as new' motor and the 125 AMP control fuse that also blew. The motor has to endure very high speeds and from time to time they do fail.
Starter motor removed