F&E tank overflowing. why ? Oh why ?

Er not quite...we're are talking about gravity circulation through the cylinder primaries.
Circulation of boiler water to the cylinder coil (often a single coil) via 28mm pipework.
Many installations barely work due to the head/horizontal distance from back boiler to cylinder.
Boiler fires up...boiler stat turns off...heat exchanger losses heat via flue and repeat.
Gradually water circulates up and around coil.
Maybe 50% efficiency at best.
Concept fine when coal and gas were really cheap.

OK I was mixing up this thread and solid fuel back boilers where failure of the electricity supply creates a hazard of the boiler creating steam if the fire is not extinquished in time

I've often wondered if an annulus was more efficient, as fitted to old gravity primary cylinders, rather than a coil. I know that when converted to fully pumped they needed to flow from bottom to top or you'd get noise from the water/air trapped in the top. Or maybe I just like the word annulus.

Update, problem solved !
I replaced the gas valve & set the burner pressure to 10 m/b (old one was set at 14 m/b.
Now everything is behaving great. ( customer phoned me 3 days after to confirm)
Thinking about this, I reckon that although everything is behaving, there must be a partial blockage somewhere in the system & reducing the burner pressure just means that it's not under as much stress now.
Thanks for all the advice.