I rang Vets Now, thanks and spoke to the guy who answered the phone. He said to keep an eye on him and watch out for any odd breathing or behaviour like yowling or distress. Since we did him, he's eaten a little, drunk and wee'd and has walked around normally and I've even had a little purr, so I hope it's ok. They also said are his gums pale (yes, but they are anyhow as he is ill). I feel such a fool.
There is one other possibility which is a worry too which is that it is hot in the flat (28 or so) and Max's back didn't get the hump from fluids because he is too hot and dehydrated so they were very quickly absorbed. The vet's cover tomorrow is a french locum. They don't do sub cuts generally in France, but I hope the senior nurse is on duty so I can ask her. If Max is ok in the morning and it happens again when we do his fluids, I'll have to sort something out.