Hi tallpaul,
I too track my repayments on my own spreadie and I am also at variance with their amounts, usually at this time of year I try once again to get them to send me specific details on how they calculate and generally I get no satisfactory response from whichever person answers the phone, usually the best I get is a letter summarising the current amount outstanding, the current repayment amount and the current rate - so basically nothing (though that is better than the year when the person answering the phone didn't even do me that courtesy). I live in hope of the day when the person answering the phone actually understands that the word "calculation" implies a little bit more than 3 numbers which I already have. I doubt even then that they will be able to help as I am sure it is in a black box hidey-hole labelled do not open on a dusty shelf somewhere in a server farm.
I have tweaked and tweaked to see if I can get it to align but so far no luck - the discrepancy is small and (currently anyway) in my favour so I am not too miffed but I feel like I should be able to accurately forecast it and am frustrated that I cannot get to the root of the difference! In their letter about rate change they have in the past sometimes stated what date it was applied, it wasn't always the precise date of the ECB change but it was generally only a couple of days difference so I think they do apply their rate daily.