Sorted out the Combi central heating.
It had a bit of a leak a year ago. I say a leak, it was probably just a couple of cupful's of water over several months, not enough to show anywhere. Small leaks can occur when the system is running, usually from radiator connections and they can evaporate without noticing, but that's enough for it to register on the pressure gauge. If the pressure gets too low the heating won't come on.
So I added some Sentinel to it. That cured it.
Sentinel is a very frequently used cure. It beats having the floorboards up to look for it and there's no guarantee you'll find one.
I checked the pressure gauge a few days ago when we put the central heating back on after the summer and it had lost a bit of water, so I topped it up. I checked it again yesterday and it had lost a bit more. So I bought some Sentinel.
It can be a bit fiddly to add, although anyone should be able to do it.
We have this tall heated towel rail in the bathroom, we had it changed from a smaller one about three years ago. (It may be the source of the leak). There are two plugs on each side of the top of the rail. I just needed the two valves at the bottom turning off and one of the plugs removing and a bit of garden hose inserted. Just a question of putting it down two or three feet, putting your finger over the end and drawing it up, whilst pointing it at a bucket and it will syphon, let it run until you've drained as much water as the amount of sealant you're going to put in.
A kitchen funnel inserted in the plug's hole stops you spilling any sealant.
A case of then replacing the bung, turning on the valves and doing a bit of a bleed.
Then the heating can be turned on when required. No more than a ten minute job.
I'd imagine you'd get no change out of £100 if you called out a plumber to do it.