That's way more reasonable than I expected, I'd looked at OpenEnergyMonitor hardware before and while nice it's not cheap.

My new boiler will be a Viessmann 100-W which looks to support OpenTherm, but some online posts say once you link it to an OpenTherm thingy you lose use of the controls on the boiler. I'll try and check that out.

Edit: Argh, nodo-shop.nl don't ship to the UK. **** Brexit.