Reasonably sure that the recent shorter weeks should have mostly balanced you out, but in any case, you are supposed to be credited any overage of work / pay and debited any under work / pay. It was released in a notice at some point.
I'd double check, but I do think for the most part, having worked though the end of year "extra" short shifts, you should be neutral right now and owe nothing, and are owed nothing.
If you've worked normal hours for the last 2 months and not been told it was short hours, I'd start making a fuss.
swift edit: to clarify, it seems RM realised that they were going to get into s**t by starting mid-year and getting more hours for normal pay without getting less hours for normal pay, so they had use on "summer" hours for a good few weeks to balance the books I think it ended the week before last?
years... 