False alarm Nigel, good spot with those vents, it's not an 'AR', not even a 'Queen Mary'. I reckon it's a TD5, certainly seems to be a route 10, so must be one of the high numbered TD5 batch. I think the TD4s and possibly the low numbered TD5s had a shorter vent than that which we can see. My money is deffo on high numbered TD5 (R171 and above), didn't some of this batch part replace the trams on 9/10.

Anthony.