If I was a suspicious sort I would think that the original intention of crewing was for you to do the last sector, then the additional sector and the standby was just a bit of creative crewing. My betting is that they had standby cover for the first 2 sectors but none to get them out of a hole at the far end.
Ultimately if you consider that you would be fatigued your requirement under UK legislation is not to fly however many hours you have flown.
Suggest you that bundle the whole lot off to CHIRP