Cricklewood ex-LNER K3 1956 what working?
 

I have a photograph dated 3rd August 1956 taken at Cricklewood, of ex-LNER K3 No 61973 "hauling up North Thames Gas Board tar empties." The loco shedplate is 32G (Lowestoft) so it is quite a long way from home. Does anyone know where this working originated and the destination of the empties, and why an ex-LNE engine on the Midland main line? Would it have replaced something else at Cricklewood to continue back to the ER?

I would like to add the photo but don't think I can as I don't own the copyright.