They have to put you in a position that you were in before the incident without betterment - hence the 3 walls being repaired and the re-tiling sections . They are probably using a general exclusion of damage caused or condoned by you the removal of the toilet to get to the leak, you condoned the this and the resultant damage.
If they have agreed settlement with you over the phone and put it in writing in full and final settlement then that's a contract!
You are right to pursue the complaints line and i would guess they they would drop an appeal before it escalates further.
Accept their revised offer as an interim payment only so you can get on with the repairs.
Oh and Slag them off all over twitter etc that usually provokes a response!