It's actually an electric furnace which heats water supplied to wet radiators (with hot water coming from a completely different electric immersion heater) - there is no gas in my rural location. It was installed many years before I bought the house and there seem to be hardly any of them around. I think it probably dates back to the early 1990s. There has been some kind of electrical overloading which has fried at least one, maybe all three, of the internal relays. There's also no guarantee that the element itself hasn't gone too. The manufacturer no longer exists and a search online for known spare part numbers has proved fruitless. So I could spend ages trying to source spare parts; and/or pay 100s for a call-out and repair fee with no guarantee of success; or buy an equivalent modern replacement furnace.
This is the kind of thing where parts continue to be available forever and they are often interchangeable (standard) so it's probably worth to keep investigating a bit more. It just depends which of the bits are broken. When you say relay, do you mean thermostat? Or is it part of the pumping system?
I've now found that the three relays are indeed fried, along with some associated wiring, but I've now taken one of the relays out so I can see exactly what type it is:
And you're absolutely right Richy, they seem to very standard and still available, such as
https://uk.rs-online.com/web/p/non-latching-relays/2452087. So, I'm off to get three of them.....
I wish I knew more about electrical stuff. And lots of other things....
... that looks like you've had a quite substantial power surge or some other over-current situation and close to a fire even, it's quite likely fuses will be blown and other electrical bits might be blown/melted also
... you need an electrician to check the wiring if you're not confident doing electrical work, you don't need a plumber
... it's odd that it has been working fine since 1990 to get such a dramatic failure, maybe you had a lightning strike?
... or if these relays are controlling the power to the main heating element then that might be blown or shorted which caused the overcurrent that fried these relays but the fuses should have blown first and protected these
.... if these relays control the circulation pump then that could be blown also which caused the overcurrent but then same as above, fuses should have protected the relays
... unless it was lightning strike or power surge from the grid (how stable is the power in your area?) then something deeper is amiss ...
.... don't listen to these guys saying to replace the wiring which is a big job for an amateur, not a quick fix, you just need to either to get this running again so you don't freeze or replace the whole system as you said earlier
... my advice at this point is get an electrician asap, doesn't need to be heating specialist even