MB: Sorry, I'm posting from Abu Dhabi and its early evening and its been a long (and hot) day so you are dealing with a slightly lazy Savoia!
I've just checked and .. Marley did add a twin to their stable in 1983 and the craft was indeed G-BMTC - bought from McAlpines in December '83 and leaving Marley's patronage in 1986.
The craft flew with several 'latter' owners including the Cambridgeshire and Essex Air Support Consortium (G-EPOL) and then, as you say, with Sterling registered back to G-BMTC.