Wonderwol,
I had this precise situation earlier in the year, and was very annoyed to find that the bank was repaying the four months or so of TRS averaged over the balance of the year. I called Revenue to ask whether I was entitled to demand return of the backlog in a lump sum, and they informed me that it is at the discretion of the bank to decide how it does so.
I took it up with my local branch - for cash flow purposes, it really didn't suit me to wait around for that kind of money - and although they tried to have the backdated sum refunded, they were told that the system is set up in a way that leaves no flexibility.
In my view, it constitutes sharp practice, but if Revenue won't call them on it, I'm not sure the regulator will either.