Balkan have a flight from Heathrow to Sofia on the 21 March: 16.15 - 21.25 and back on 28th 13.55 - 15.15. This flight is also direct (no stopping off at East Mids). Also Mourgavets available, cost for 2 is £741.85 and FREE LIFT PASS or half price ski pack.
I booked this morning, 21 March from Heathrow staying at the Perelik. Going on my own this year as can't take 14 year old son out of school, so wife and him staying at home. Are you still flying from Bristol or have you all changed to Heathrow?