@GavinC, I pulled the vacuum hose off and smelled it, but I didn't realise the ignition should be on, presumably to keep the fuel fully pressurised. I'll try that next.

@Gpl1968, I've checked the fault codes most times the light has appeared, and it is always 1-2-1, which Haynes reckons is "Heated oxygen sensor signal missing or out of range".

All, the garage that did a pre-MOT for me sold me some Forte "Advanced Formula Gas Treatment", which I've used. Since then, so far, no issues. Even the lumpy idle has improved.

On the FPR point, and the smell of petrol, I've had the car out for a few good runs, and a couple of times deliberately left gear changes a bit late so that the engine was getting slightly punished, and then immediately hopped out the car (having parked it first lol) and sniffed about for petrol, and whipped off that vacuum hose. All this is since I put that gas treatment through, and I can't find any cause for concern. The thing is though, faults don't fix themselves, so either that Forte stuff is extremely good stuff, or its just the randomness of the fault is trying to catch me out again.

The oxygen sensor is the Lambda sensor. It provides feedback so that if the mixture is too rich/lean the fuel system can compensate.
Assuming that the electrical connection is ok then you probably need a new Lambda sensor.
The Forte stuff will help by cleaning the injectors to some extent, but if the Lambda light comes on again, you need to bite the bullet and get a new one.