A simple 240v 1 hour timer push button switch

Put in a PIR switch connected to a thermostat and then a motorised valve. It will switch on when you enter and switch off when you leave. The thermostat will stop it switching on if its hot.

Be quite easy to make, if you have reasonable electronic skills. Using a

Having to stay in the kitchen, dancing a little jig to keep triggering the PIR might be slightly less than convenient while waiting for it to warm up.

You sometimes see light switches like that, but with a shorter delay.

Radio Spares (and do doubt others) sell 'delay off' relays. These relays close when power is supplied and then stay closed, possibly for a variable period, after power is removed.

I like that as a solution but would mean that the heating kicked in even if you only passed through the area rather than intending to remain in there.

One could easily have a long hang time after being triggered but that wouldn't work very well with the passing visit. Warm up will be the same in both cases of button push or PIR trigger.

You can get timers. I doubt the cheap air operated ones would go out to hour reliably and are a pain to adjust even at a few minutes delay. So it's DIN rail modules which come in all manner of options...

yep, but doing such little jig will warm you up anyway ;-)