Boiler is not turning on - Water Heaters

It is no flame detected. if gas meter was changed boiler should have been turned off . If this was not the case you have air in the boiler line which is try to purge out by the pilot which is a very small hole so this would take for ever .

Turn boiler of to try and reset it or you will have to purge boiler gas line .in the boiler .

Did the boiler winter over with the heat turned off and no winterization? If so, then the boiler may have frozen and cracked. Replace it.
If it WAS drained down and winterized, and you just filled it with cold water before firing it, it may be condensation.

Combi-boiler-turn hot water tap on/off and the heating stops for ...

singletrackworld.com/.../combi-boiler-turn-hot-water-tap-onoff-and-the-...

Combi-boiler-turn hot water tap on/off and the heating stops for 20secs ... hot watertakes priority over central heating so when you run the hot tap, heating is ... Yeah onmy worcester combi there is a diverter valve - it feeds either the heating or ...

My central heating works but I have no hot water - 24'7 ...

https://www.247homerescue.co.uk/my-central-heating-works-but-i-have-...

Diverter valves switch heat between the hot water and the central heating. When youturn your hot tap on, the diverter valve temporarily shuts off the water supply ...

Does your boiler keep switching itself off? Here's why ...

www.dhs-uk.com/does-your-boiler-keep-switching-itself-off-heres-why/

Jul 7, 2015 - ... Bathroom Suites, - Showers, - Wet Rooms, - Taps and Accessories, Information and ... Low water pressure can cause your boiler to turn itself off. ... boiler is not turning off, you aren't getting the amount of hot water you require. ... If there is air in your central heating system, then your boiler may shut down.

If the gas has been turned off for awhile,the unit will have to bleed the air from the boiler before it is lit.

It could be dyslexia, but I think you want to know what may be wrong with your boiler in this case!
This is a VERY old design. The diaphragm is on the water section, this operates a switch to turn ON the pump, and a further device causes the diverter valve to divert boiler water from CH duty to HW duty.
From your description it appears that the boiler is shutting down on internal boiler water temperature (too high), your 'briefly too hot' HW symptom may be caused either by the boiler side of the secondary heat exchanger being blocked/caked in sludge, OR the DHW side of it being scaled. Either way a new or replacement (descaled) secondary heat exchanger MAY inprove things. It's not easy on these old clunk-pots, and EVERY disturbed seal will subsequently leak, so get a good supply of o-rings and fibre seals before you start ("you" being the competant fitter you employ)

Lack of pressure in the primary circuit [radiators] Look for a pressure gauge should read between 0.5 & 1.5 bar
Somewhere there is a tap/valve/pipe connection to refill & looks like a small thumb turn or a screwdriver turn on a connection on the mains water inlet pipe. turn on until you get the right pressure then off

Water logged expansion tank ( ruptured diaphram in tank) or wrong size tank. This caused the excessive pressure in the system causing the relief valve to open. Also, air in the system causing flow to stop, Both conditions will cause boiler go out on safety.

Your boiler is on, re-check t-stat. Turn breaker off and rad and boiler will cool.