VMT for your replies. I got in touch with Pre-Travel Support and they were everything GoldCircle said they would be. They took down various details and then got back to me promptly explaining that there were 3 issues at play:

1 - The specific fares, terms and conditions posted on the Friends & Family section of flybmi.com

2 - The fares actually bookable on the F&F section of flybmi.com

3 - The fares loaded into BMI's computers

All 3 were (and still are as of today) completely incongruent. I was informed that thanks to my innocent query BMI will now try and remove all the fares in question down to the lowest common denominator, which from what I understand will leave one I class fare LHR-CAI, which seems quite random. (As of today the route in question is still bookable online with a Saturday night stay.) They cancelled the original booking and did not offer to accommodate me in Economy. However one of their dummy bookings was left active and emailed me randomly reminding me I'm due to travel soon, apparently booked in G class. I will have to ring them yet again to find out what's going on. Thanks again for the help though, it was better to find out these things quickly rather than close to the date of travel.