Thanks to a post on Bob Cordery's excellent Wargaming Miscellany blog I was alerted to a deal at the Flying Tiger chain of stores.
I'd never come across these before but the seem to have stores all across the UK. originally from Denmark they sell a wide range of Ikea-ish small craft and household items.
The one that had caught Bob's eye was a magnetic pad, I guess designed to stick on a fridge etc as a wipeable memo pad (it comes with a pen).
Conveniently these are A4 sized, so ideal for lining a box of toy soldiers! The pads retail for £2 each so a pretty reasonable price. As there was a shop in Kingston I hopped on a bus and picked up a few.
Worth picking up if you have a store near you.