Course you see why there's a need to leave the tap open. Your leak.
Water regs require an internal double check valve on new installations.
Go on admit it, you did it yourself.
Should have an isolation valve and an internal double check valve. Isolate I side, open tap outside, removes most of water and prevents the ice pushing off compression joints.
It's a sealed system so if its leaking from central heating side it will stop. Easy to spot as pressure gauge will be showing zero.
If not its on the domestic hot water side so if theres no isolation on cold water going into boiler, normally a valve under boiler, then turn off main stop tap and...
By regs if you are going to continue using the fire then it should be removed.
In reality pipes are cut back as far as possible and the back boiler drilled. Light a fire and you will get some steam as the remaining water is boiled off.
Turn oil boiler stat down to 60 as a starting point.
It will have ran like that for years. If you need better control its going to need £££ spending. Depends what your end game is.
Sounds like a mectron burner. You can hear it go through the ignition sequence and definitely no oil at point of ignition and locks out.
I'd first look at oil pressure whilst washing and also photo cell.