Thanks for all the replies. The website said 30 minutes and it still says 30 minutes so they don't have a leg to stand on in my view...:
https://www.flybmi.com/check-in/en-g...r-booking.aspx
Unless they have updated it and I'm looking at a cache of the old page...in any event, I have taken screen shots in case it is updated (but presumably they would have a record of doing this).
Several BMI and BA staff members were also very rude. I'm tempted to "name and shame" one of them in particular who accused me of lying about what the BMI website said.
To make it worse (i) the check in desk agent refused to ring the dispatcher at the gate for 10 minutes - if they had rung straight away I would probably have got on the flight, and (ii) the check in desk agent also refused to print my boarding pass even though I had already checked in online - if he had done this I could have abandoned my luggage and still made the flight. A BMI supervisor then told me they were all losing their jobs and didn't care any more about passengers making their flights or not. It was a total omnishambles. I've had bad experiences with both these airlines in the past, but nothing so bad as this!