Just thought I would update you all - bmi have redeemed themselves again !
Had some spare time at lunchtime so gave them a buzz just to see what happened, and after some time of being put on hold and offered a re-routing on LH via FRA (which would have seen me arriving in NAP 10 minutes earlier than the bmi flight !) I was put on to an extremely helpful English agent/supervisor who was in India but originally from the schedule change department and she endorsed me to my desired BA flight with no questions asked.
I even said that I knew they didn't even have to refund me but that I would rather just have the outbound sector endorsed rather than be refunded and re-book on BA, and I told her that with a restricted ticket I wouldn't expect to be endorsed.
Am now v. happy and looking forward to sampling club europe, only string is that I have to leave from Gatwick rather than LHR, but no big deal.
Just goes to show that even though there are still some customer oriented stars in bmi who will go the extra mile just to prevent a small inconvenience to a passenger.