Right, that's the Brompton added to my home contents insurance for cover away from home (it wasn't covered otherwise). Turns out adding it to the Direct Line policy was cheaper and had more flexible conditions than specialist cycle insurers. That was about £3.60/month for £750 of bike, which seems to be a reasonable balance of cost and risk to me.
I've also decided that any cable lock is just a bad idea, so despite the inconvenience on a folder, ordered a top rated U-lock
. (I've always used a U-lock on my standard bike).