This field could and does show every time of day, up to and including 28:00 (04:00).
For all intents and purposes, even though a 28:00 is the next day, my other field is just showing 04:00 with no day marker, so I want these two fields to be the same (day can be dropped).
I then want a field which will calculate the difference between the two fields. and for that to show up as number of hours and minutes.
For example.
15:00 and 25:15 would be a difference of -10:15 hours. However, with the formula you provide the 25:15 will be converted to 01:15
15:00 and 01:15 need a calculation for the difference between these, which is 13 hours 45 minutes (13:45)