MIBI don't step in like magic, Ken. They appoint another insurance company to act as the insurer. The first thing the appointed firm will do is refute liability and drag BJP/Axa through the courts screaming and kicking to ascertain proportionality... in practice there will be no dragging, screaming, kicking or apportionment, Axa will accept liability and load DMs next quote to the hilt.
I'd take the broker's advice, DM. Despite your suspicions he know's what he's talking about... and stop worrying about it, FFS!