its not that simple,
they cant just change the number.........
if only it were so easy, like people expect it to be.
It sounds like bad system set up if you ask me.
You set up a key based on:
Single or joint
FTB or Non FTB (can be based on actually be start year with some manual overrides)
Start Year
This will give you the rate and the max relief available which can be applied to the annualised interest (which will already be calculated elsewhere).
The problem is that IT departments in every large organisation are unfit for purpose. Going back 10 or 12 years something this would have been coded, tested and implemented by a small team in less than a month. The issue now is that IT areas have been flooded by people with no discernible skills.
It is one area of evolution in which we have regressed over the last decade.