I am not going to consider this a spoiler because it was already made very clear at the beginning of the show. Higgins and Robin Masters are TWO different people.
I didn't catch this first time watching through, probably have something to do with it being once a week for 23 weeks then repeats before the new season, you still tend to forget things. But this episode starts the (in retrospect) very WEAK subplot of "Is Higgins Robin Masters?"
Well considering in the first Season Thomas specifically said he MET with Robin Masters, and on several occasions spoke with him on the phone while Higgins was standing right there, and there was one in season 3 where Higgins and Thomas are standing there as Robin exits a car (Played by the amazing and talented Orson Wells).
I imagine Wells' death may have given the writers this idea to create this subplot but they should probably have checked back to the pervious episodes. Anyway, either way there is too much evidence including other guest stars who had personally MET and KNEW Robin. Also if you are thinking, 'well he was a stand in' There was an episode where Robin Masters was at a book singing and was met by the guest star, so they SAW the (Wells) Masters character and got their autograph and it was definately NOT Higgins.
Anyway still a good episode, but as the other guy said, I wish they would do more with the "Lads"