After about 40 minutes of simmering it on the hob with the lid on and an overnight chill in the fridge I ended up with this....
Which crumbles into my hair and then I can't get it out, and which, as my mother rightly pointed out, I probably used more energy cooking it for all that time than I saved by reusing my shampooy fragments!
But! I wasn't ready to give up on saving my little bits of soap and shampoo bar, so after much google searching I have decided on this method to reuse them both...1. Find an old sock or cut off the foot of an old pair of tights (I have gone down the tight-y route this time as I just so happen to have a bag full of old tights I am also hording and hoping to reuse!)
2. Every time your soap or shampoo bar gets to the too-small-to-be-usable stage put it into the sock or tight-foot.
3. When the sock/tight-foot is full tie a knot in the top of it to keep the soapy bits in.
4. The sock keeps all the bits from disintegrating on to you, but you can still lather the soap through the sock, either directly onto your hand or body or onto a sponge or whatever else you use in the shower.
I haven't tried it out yet because my tight-foot of soapy bits is nowhere near full enough, but I think it sounds like it'll work! There are lots of other ways of recycling soap scraps all over the internet, but the way I'm using seems the easiest and the the one that uses less energy.