Richard,
Whilst I see where you are coming from, as you have put tighter time limits than are actually accepted by the industry, you won't be able to get it much better.
You can be more than 15 mins late leaving the gate due to inconsiderate passengers who like to stay in the bar too long before going down to security, holding up the other passengers, this again is not the airlines fault.
The point of departure is the gate as no airline has contriol over the waiting time in the stack for take off.
The only truly accurate figures for Take Offs and landings are the ACARS figures from the aircraft and ATC, gatwick and other BAA airports can be innacurate with their information.
In my opinion, anything within 1 hour of actualy take off time is acceptable, as its an airline, not a production facility, or even a TV programme
By the way, the industry figures show the VS27 on time 51% with an average delay of 51 mins