MelF
I had this issue a few years ago.
I also usually pay in full every month but a few years ago had a particularly large bill, about 4000, so I paid about 3500 as I would be getting a refund from elsewhere for some of the bill so would pay it next month.
I was disgusted when I got next statement, they charged me interest on the entire 4000 and not the 500 balance.
I phoned to query this and they helpfully just posted me a copy of their terms and conditions. I asked if I paid 3999.99 what would the story be and they replied that I would be charged interest on 4000 and not €0.01!!
I am glad to report that I wait till the very last moment every month to pay my bill to take advantage of interest free periods and they have never earned another cent of interest from me since than and nor wull they into the future.
They are basically one step above moneylenders with policies like that.