I think "Q" is added to a flight number to show an amendment or change to the original flightplan. Usually, its for a change of route or tech stop. However, in this case ATC may have requested a change to the callsign to avoid conflict.
Trust me - this is wrong. You will find quite a few of the regular LS schedules have a Q added - nearly always when you have similar call signs going to similar destinations, at the same time - eg Leeds, Manchester & Newcastle flights ont he way to Tenerife within a few minutes of each other.
It has nothing to do with an amended flight plan.
TSR2 - you are welcome.