Just an update: I got an itemised bill showing the fax being sent with the correct date and time. I sent a fax to AIB outlining my complaint and all of the details. I got a letter yesterday to confirm they have changed my mortgage to standard variable -€300 per month saving!!
I was kind of hoping for an apology too though - In 18 months AIB have (1) taken approx €1,800 from my current account to repay mortgage even though this isn't the repayment amount and the normal repayment was made. They refunded this within a few days (after a few phonecalls and e-mails).
(2) Charged the incorrect rate from last April to October. Again, after many phone calls they refunded money.
(3) As outlined above they 'lost' a fax requesting a change from Fixed to Standard variable.
Do other people have as much trouble as this? It seems totally unacceptable and I will be keeping a very close eye on my statements from now on.