Mr_Numbers,

Not certain where I saw this and it is possible that I confused it with Peak Vehicle Requirement as Baboonery notes.

However, there is this source:
http://www.londonbusroutes.net/details.htm

As well as PVR, this also has service frequencies (see also the column explanations). This should make it possible to calculate the total number of services per timetable.

As you say, a good start hopefully.

HTH