T i m wrote
Rod Speed wrote

Any ideas? Is it time to "get it looked at"?

Could be muck in the fuel tank blocking the outlet.

That would see it do the normal grass fine and
not the long grass which needs more fuel.

Or water in the fuel.

Ditto.

Or aprblem with the valves (do compression check).

Ditto.

Except, things like that can start to be a problem
*once* the engine has fully warmed up.

But he didn't say the problem is only seen when it has warmed up.

And has now added the crucial information that it misfires all the
time and only stalls when used on light grass, not on heavy grass.
Your possibilitys don't explain that behaviour.

Further, our car would start (/restart) when hot
or cold, not when warm, but would run perfectly
well at all temperatures, including warm.

Not relevant to his symptoms.

I did sort a motorcycle for someone that gained an airleak on
the induction side on one carb once warm, only after the local
bike shop had charged him to strip, clean and re-jet the carbs ...

A batch of 4/ strimmers were losing power when run hard for
a long time but not if only run hard between periods of ticking
over. That was down to the valve gaps closing up ...

Again, not relevant the symptoms he has.

With any fault finding, you have to consider if a particular
potential fault could produce the symptoms he is seeing.