The desire for new tubes was caused by swapping from winter tyres (Conti Top Contact Winter 37 (actually 31)) to summer tyres (Voyager Hyper 37), which revealed that I'd had my first puncture of the year a couple of months ago.
I'd been having to pump up the rear tyre every 2 or 3 days. I'd guessed there was probably a thorn in it, but an external inspection showed nothing, so I didn't find it until I changed tyres and looked on the inside.
The punctured tube was a Vittoria 700x38-45, and I figured that continuing rideable for a couple of months with a 1 cm thorn through it showed that it was a good tube, so I went looking for more, and was pleased to find I only had to pay 99p rather than a more normal £4-£5.