Bren,
I always thought the bean-counters must have been livid when they saw how many extra bums in seats were required for the various 'splits'.
If IIRC before the split LL O/R was 75 valid ATCOs plus the hangers-on, to achieve the split required 45 TWR & 45 APC, an increase of 20% and 5 of the latter received promotions to be Group Sups. If the figures were the same for KK & SS that must have meant another 10 or so ATCOs.
I expect the same happened when LATCC split into Terminal and En-Route.
Personally I thought it was a complete waste of time and money to put the Approach functions into West Drayton, and I can't believe management in their wisdom were looking ahead to when, in LL's case, there would be a new tower - I think work on the latter ( regarding furniture, manning etc ) didn't start until later in the 90s.