Cheers Cowley.
I think it's fair to say that 37901-4 were pretty reliable. The biggest problems I can remember, and as you say, it's a long time ago, were with the radiators and silencers.The radiators were made by a company called Spirol Tube and supplied by Brush Traction who project managed the conversions. They seemed to be constantly leaking and I got sick of changing them. As far as I know they're still leaking!
The silencers were made by Gloster Saro and had to be a very complicated shape to fit in the available space, bearing in mind that standard 37s don't have silencers. They were made from Spanish stainless steel of dubious quality and cigarette paper thickness so they were very prone to cracking and blowing their stuffing all over the place. They were chucked out in the end and the loco's reverted to a straight-through exhaust.
The engines themselves were pretty good, the odd leak aside, and if Class 38 had gone ahead I think it would have had 6MB275T engines.