Thanks for these replies, it's as I thought.
I've had the IOM Govt as well as the British Embassy on the phone, they have spoken to the macho men immigration here, who aren't budging.
I'm being deported on the next flight to Gatwick apparently.
Total nightmare. Hopefully doesn't happen to anyone else but their sheer incompetence of not recognising 'British Islands, Isle of Man, as a British citizen's passport has cost me my holiday.
Guy at the Embassy said if they don't like the look of you or the passport there's not much they can do, it's their power and prerogative. And that I have just been unlucky with officers on duty at this particular time.
Might speak to IOM newspapers on my return to drum up a bit of awareness.