On Wednesday, 9 January 2019 10:29:40 UTC, Nick Odell wrote:
Cooking on the gas hob last night, suddenly the flames began to flicker
yellow.

First thought: dirt on the bottom of the pans - so I turned on the other
burners and they were doing the same.

Second thought: low oxygen (panic!) but even though I felt no personal
effects and the house is old and draughty as hell, I opened the back
door and continued cooking with the open air blowing into the kitchen.
It carried on like that for another few minutes and then it went away
leaving only normal, blue flames.

Everything is AOK normal this morning and it's never happened before so
is it reasonable to presume there was some contaminant in the pipe? If
so, could it have damaged my combi? Next combi service is due in about
nine months.

Nick

Probably something you've spilt on the burner evaporating and burning off.