Mortgage Form of Authorisation (lost)

My fixed rate mortgage expired about a month ago and my lender sent me out the usual letter advising of same and an accompanying Mortgage Form of Authorisation (MFA) with my options for a new fixed rate / term etc.

I filled it out and sent it back (opting for another 1 year fixed) along with the relevant details (BER etc.)

Couple of weeks later I get a letter from the bank to advise that the previous rate had expired and I had been moved to the (default) variable rate (i.e. not what I had requested)

Rang them, they said they had no record of any MFA being received which was why they defaulted to the variable (which, to be fair, is precisely what they say they will do) Offered to check with their mailing department to check if the form was stuck anywhere but, after several weeks, they have again confirmed that no form was ever received.

Have corrected the rate now, digitally, but am still out of pocket by several hundred euro per month as the difference between the new rate and the variable was a fairly whopping 0.8%! So have effectively been overpaying the interest for over a month simply because "somebody" lost my form.

Asked if the bank would consider back-dating the rate to the end of the previous term, they said they couldn't because they have no paperwork to back up my claim. To be absolutely clear, I posted the form in my local post office (not an on-street post box) and well in advance of the required date.

So it looks like the post office have lost the letter and now I'm out of pocket to the tune of several hundred euro as a result. Didn't send it by registered post as didn't think there was any need to (just a form, nothing sensitive, and sure everything sent by post gets delivered, right?)

Really more of a rant more than anything else but it's money I'd rather have in my pocket than someone else's and it pains me that I did everything right (bar not paying the fiver for registered post) and on time yet I'm the one getting stung.

Have I any recourse with An Post here or do I just have to swallow it? I've no doubt if I even bother complaining to them they'll just shrug their shoulders and say if it's not registered, they've no way of tracking what happened to it.