To replace one of my bikes would run to about 5k ish. Has anyone got any experience of telling M&S about an expensive bike they want to cover and how much more its likely to add to the price? (Given that I currently pay 450 ish for build/conts insurance with M&S )

Yes. I’d be inclined to “self insure” the difference if you’re not planning on them getting nicked. You’ll save the difference in about 3 years IIIRC.

S works epic added about £200 😳 still covered away from home etc and wayyyyyy cheaper than a bike only policy.

Mine would probably cost +£4k to replace, but happy to get £4k back at little cost – standard M&S for me.