Ok, so now I've read that it may be with canned fruit, dried fruit/baking, or near the ice cream in the freezer area. Phew, I'll be going on quite the hunt when I finally drag my butt to Waitrose (Sunday maybe?).
Also, you can use butternut squash in place of pumpkin, if you're cooking it yourself. The flavour is pretty close once you add the seasonings you associate with most pumpkin dishes (nutmeg, cinnamon, allspice, cloves) and there's a better flesh to skin/seeds/emptiness ratio. Besides, the pumpkin Libby's uses is of the same species as butternut squash, but a different species to jack-o-lantern pumpkins. But it is easiest right out of the tin.