Osgood,
I have been to Middleton and measured up Mary HC D577. They were very helpful, but the loco was in service and so we could not climb on her.
Also Statfold Barn Hunslet Archives have sent me a scan of a pipework drawing of D621, an 0-4-0 allocated to an RAF Ordnance depot. They had no General Arrangements of the early DM 0-4-0 locos. This gives the overall dimensions, but omits the front after the radiator.
I have a very poor photo of an identical loco from the same order: D622 at Wilford Ordnance Depot in Nottingham. D621 was a 100hp machine, whereas Mary was about 200hp, but the general appearance is about the same, although the cab was 6" shorter according to our measurements.
Colombo