Thanks, all!
You're definitely right about the window mounted antenna. Mine will get a signal if I'm parked within 500 metres of the transmitter mast; otherwise it's useless. I've soldiered on with it, but a new DAB radio has necessitated a change.
ianmayco68 - this is a possible solution. The chap fitting the radio suggested fitting something similar to one of the back corners of the roof - so it's out of the way, and less likely to get damaged, but will still poke through the roof rack to get a signal.
If I can get the shark fin-type to work, that would be my preference. Neater, and it won't get damaged by me fiddling on the roof. It's probably just a question of where to mount it, that's clear of the roof rack. Maybe on the back?
Best regards,
Otto