I was not even aware of 31101's infiltration, and was also a Controller in Glasgow, from 1984. The only 31 I ever had on the WCML, unaccountably I did not record the details, was when a Down freight damaged the overheads somewhere between Carlisle and Lockerbie. The only loco available at Carlisle for an Edinburgh-bound train was a 31, which worked the train (slowly!) via a reversal at Carstairs, from where I took a bus to Lanark for a train home towards Glasgow. I'm not even sure what year this was, possibly 1986-87.