Going back to the original post:
"In fact we took off 45 mins late but arrived only 15 mins late!"
Don't forget that airlines build in a lot of time to their schedules to allow them to arrive "on time" or "early" as often as possible.
Between 15 and 25 minutes is regularly built in at Manchester for taxi times once the aircraft departs the gate. If you can get airborne quickly, you can be sometimes be 10-15 minutes ahead of schedule as you are taking off - even if you departed the gate at the time stated on your ticket.