Has it been doing the Lymington branch or the Waterloo-Exeter route?
Initially it was being kept to the Salisbury 6 route, but it has been noticed working singly on the Salisbury to Waterloo stopping services and is now on the Lymington Branch as of this week.
The 101s had a BUT (AEC / Leyland) engine, which was basically a contemporary bus engine. Not certain, but it may well have been a Leyland O.680 in the 101s, which you would find in many Leyland buses.
Indeed they were, there is little difference between a rail mounted and bus mounted Leyland O.680 Engine and initially the earlier DMUs were powered by BUT (British United Traction - a joint venture between AEC & Leyland) engines, before the parternership was bought out by Leyland Motors who continued production until the advent of the TL11 Engine - Which also found use in buses such as the Leyland Tiger and the Class 140-144 Pacers.
Which is why GBRf and EMD did the silly thing where they built loads of engines for the 66s, shipped them here, sent them back and are now fitting them, because technically the engine is still fine. Does that mean that you could in theory design a new DMU compliant with latest group standards, but use an old engine that can be obtained in large quantities to power it - or do the latest group standards prohibit that?
That said, GBRf & EMD were at one stage looking into the possibility of constructing more 66s over here at Doncaster Roberts Road, and having the engines over here would be useful in case of a 66 requiring a spare engine before finding the loophole and sending the spare engines back to EMD.
Going back to the this thread though, I'm surprised RVEL are carrying out the work, I'd have thought Porterbrook would be working with Wolverton or WABTEC on this as apposed to RVEL who as of yet, have had little passenger rolling stock work.