The following interesting points emerge from this analysis:
there were 13 to 14 vehicles out of use for around a year from mid-1958, mostly due to the gap between the end of the Central Merton allocation and the start of Dalston. There were also typically 4 vehicles out of use all through 1963- I‘m not sure why that was. The 1 or 2 vehicles out of use at many other times was typically due to garages waiting until the start of the next calendar month to re-licence a vehicle that had returned from overhaul or storage, to economize on vehicle taxation.
The overhauls graph shows that there were no vehicles in works for around 5 months in 1957, presumably because they had all been done in 1955-6. More consistently, there were usually 2 or 3 vehicles (and never none or more than 4) in works all the way from mid-59 to the end of 1965.