There were cellars under the offices at the corner of Mill Lane and Digbeth - I've been down in them. They were used for storage of old waybills, etc. and accessed from a manhole in the former GTA's office (GTA=Garage Traffic Assistant - the chap who did the daily rotas)
There were bus parts put in the cellars of the former houses in Bradford Street - the area was later used for a staff car park, then tarmaced for coach parking in the 80's or 90s. The area between the entrance to the fuelling bay and the Anchor pub.
The former dock foreman, Norman Keen, told me he had emptied redundant parts from the depot stores when the houses (which he called 'The Cottages') were demolished and put them in the cellars, the remaining space was filled by demolished brickwork.
The view of a 'cellar' in photograph
https://my.bus.photos.fotopic.net/p47860081.html was actually part of the engineering pits area, a sort of parts and tools store. The pit support beams can be seen extreme right, with surrounding smooth-walled areas.
This area had perviously been 'the patch' used for bus parking, as seen in some of motorman-mike's pictures. It had been cattle pens for the markets before that.
A single house had stood about where the mechanical digger is in the photo, possibly to do with the cattle pens originally but owned by the Midland Red later. I don't know what it was used for, or if anyone lived in it in its later years, it was demolished about 1973.
