Try this one! Ferroli Optimax 25ov?

Boiler will not start after programmed off for 6hrs plus in cold weather. Once boiler has fired after resetting a number of times it will run 24/7 so fault is difficult to isolate.
Running on LPG, spark, gas pressure, gas valve operation and CO2 all ok.
Boiler will fire up after 3 or 4 hrs programmed off with no problem.
Gas valve opening each time.
Possible clue, by loosening off the injector nut the boiler will fire, the nut is retighted and the boiler runs happily until another shutdown period.