Had a call from a tenant of mine last weekend to say that their heating was out; I went round and found it was caused by the combi boiler having become depressurised. I quickly refilled it, and showed them how to do it, but with a strict direction to call me if it happened again.
Bugger me, they've just let me know it's happened again, so clearly there's a problem which needs sorting out. I'm going round again tomorrow to have a closer look.
My good news/bad news is that I have a service contract on the boiler but not on the radiators; ie if it's a leak somewhere in the CH pipework then I'll fix it myself; but otherwise the engineers will do it. I'll check for leaks tomorrow, but think it's the less likely cause - the system has been running fine for years and hasn't been disturbed, and unless it's in pipework under a floor I'd think a leak of this size would have been fairly clear. I'm hoping expansion vessel? Boiler (Sime "Friendly Format") is probably about 15 years old and it hasn't gone before.
What I'm asking now is for any tips I can look for tomorrow (in the absence of any obvious leaks), really to nail down whether it's the boiler or hidden pipework at fault, so that I can phone the engineers on Monday knowing what I'm talking about!