I have my computer set to download upgrades at 00:04 (local time) on Wednesday morning to take advantage of the unlimited overnight usage but still be early enough to switch off afterwards. Last night it downloaded 22MB, but my overnight usage shows just 81KB. Is midnight to 8 am GMT all the year round, or could there be some other explanation? Obviously, there's little point timing activity to happen in the unlimited period if it's not being applied accurately at your end.
BT only report usage at intervals and all usage in the interval straddling midnight is discarded because the pre- and post-midnight distribution can't be determined. It's possible your download early this morning fell into that category. The midnight to 8AM period is local time throughout the year. I assume that means we lose a "free" hour in Spring when the clocks go forward, but gain one in Autumn.