I had the same problem a few years ago with Thomas Cook; we booked for end of August on January 2nd.
They said the others booked before us (we couldnt prove it and as it was 2-3 weeks before we left, we couldnt find anything else that would take 8 of us together so had to have their offer of 3x2 bedded rooms with extra beds in 2 of the rooms. It either meant my 80 yr old mother sharing with 2 15 yr old girls or the 15yr olds being in a room alone. We had no choice or no holiday and the rooms were so cramped we ended up with bruises all over our legs from where we kept banging them.
ABTA couldn't do anything without you paying to take it further!!!
Good luck with your fight!