Hi Nige,
I have 2 inputs

1) my understanding was that First Ireland would quote you on BMW if >15 years. Did you make sure it was for a classic policy / talk to someone knowledgeable ? I would check it again.
contact details :-
email: clientcareschemes@firstireland.ie
Barry Campion APA (Personal and Commerical insurance)| Client Services | First Ireland Risk Management
15 Parkgate Street | Dublin 8 |
telephone +353(1)882 0849

2) Autoline/Newry will quote some marques (eg Porsche, not sure about BMW) down to >10 years old. I am looking into this option now myself so that is concrete.

Good luck with the quest... MM.