I went through all this nonsense with HMRC a few years ago..nightmare, but in a nutshell they "advised" Days spent in UK that "count as work" are based on time, location and circumstances. For instance, if you "transit uk", remain on A/C on the ramp if its less than 3 hrs its not counted as work in UK. If you Divert to UK and remain in A/C for less than a "DAY" but more than 3 hrs, would be treated as "exceptional circumstances" and not treated as work. If you do layover in UK, get of A/C for more than 3 hrs, you are subject to the "Day Rule" midnight/ midnight and it counts as UK work. As mentioned, its not that easy as the Statutory tests" also look back at your UK residency status in the past 3 years. Therefore, if you do a 36 hour bullet stop in LHR its a day and a half towards your allowance. They will ask for copy of rosters as the defining article. Having said all this, they seem to be on our side, and "generous" with the calculations, for instance if you land 23.00 local you could argue your duty finished 00.01so that "Day" is disregarded as its after midnight.