Hi, thanks for your reply. Lufthansa website didn't like my reference number but www.bmiregional.com did take it and said I could check in online 48hrs before. I phoned bmi to check whether seats could be pre-booked but they can't, it's done at checkin. Likewise supplying passport data. The agent also clarified that online check in was ok for bristol to frankfurt, but can't be done the other way.
Thanks again for your pointers.