Sorry, M, no one is trying to trick or belittle you. Are you new to plumbing, or just to Gas Safe? As per LP, how do you know there is good flow, as I thought no heat is leaving the boiler? You cannot "feel" the flow, apart from assesing temperture rises either by your hands or thermometer. You may feel some vibrations, but that does not mean water is circulating. You need to start from basics. the boiler is tripping on the 'stat (Yes, apparently). The 'stat is reacting to a rise in temp (sometime the stat or connections can be faulty, but in thos case you are reporting kettling (which, BTW, it probably isn't, it is overheating, which is slightly different), so we know the temp is raising at the boiler, evidencing that the stat is doing its job). That bit tells us there is a circulation problem - some sort of blockage/restriction. The fact that you have no heat to rads or hot water tells us that the problem is before the motorised valves or after the junction of both circuits, forming the return. (Do not start removing rads as per one of your early posts, that is a waste of your time.) The list of available suspects now becomes very limited. It boils down to: air, sludge or scale, or a valve
Air: are there any manual or Auto Air release bleed valves on a high point. Typically immediatly above the boiler, or in the A/C. AARV are notorious for sticking or leaking. Quite often, they are connecte with a compression nut - so you can carefully unscrew the nut below the valve to check for water/air
Valves: These are often inadvertently turned off, (or turned off, back on again - but without the spindle turning. Often a punter in a blind attempt to sort a problem will try various knobs) so you need to check. (I am aware this seems unlikely due to the occupants, but could a "freind" have been trying to help?
From experience: If there has been DEFINITELY no prior attendance by an "engineer", the A/C will be the location of such a valve problem, If there has been another "pro" in attendance, look at the F&R valves on the boiler. This is a prime area for a lever valve to be apparently in the correct position, but the valve being stiff locks in the closed position. A bit of effort moves the plastic lever, but leaves the valve closed. the same applies to Mag Filters.
Sludge or scale:. This, to the extent of a complete blockage, is actually very rare: I cannot recall seeing it occur, (on a heating system) TBH. EXCEPT (at a grave risk of being accused of repeating myself, at the junction of the cold feed and the system - have you checked this???)
You must relax, stand back, and apply logic. Talk to the occupants, (more often than not, I can ask 5/6 questions and be confident where the problem lies), but be aware that they are sometimes stupid, and sometimes lie
Years ago, I fitted a sytem in a "standard" house that was being renovated. Ages later, I was told a bedroom rad was slow to heat. I could not work out what the problem was, but it was no major issue. Until another worker lifted a floorboard, pulling out a nail that somene had hammered in (obviously never having heard of "pipes in the centre, nails on the edge"). It presented enough of a restriction to reduce flow to that rad, but had NEVER apparently leaked. Looking back, if we were all concerned enough to find the problem, logic would have forced us to find the problem.
Good luck
