I’ve not actually had the time to investigate (and haven’t been brave enough to tweak the ‘new style’ Shimano front mech while out on the road…) but when I’m in the big ring and say 2/3rds of the way up the rear block (say 3rd or 4th biggest sprocket) the chain starts to want to fall off the inside of the big ring.

Obvious place to look would be the limit stops, e.g. if the front mech isn’t moving far enough outboard, but I wouldn’t have thought you needed to rely on the front mech to keep the chain on the big ring in those gears? It’s not like I’m even properly crossed up (I spent my formative years riding with my dad behind me shouting at me if I ever went big-big…).

Could it just be worn? 5000km on the clock…

Ta

Possible rear mech issue? If the cage is out of alignment then the lower jockey wheel could be pulling at an odd angle.

Good point, but happens under power only (in fact, especially under power).

Which I guess makes it more likely to be a chain wear thing than an adjustment thing…

sounds to me like the front mech cable needs a tweak to tighten it up slightly.

Could it just be worn?

Yes. I had exactly this recently, and it was just worn. The chain was well over 1% and it had knackered the chainring too. The chain had done less than 2,000 miles, although winter bike so only used in bad weather.

I think this was only the 3rd chain that this chainring had seen, but I allowed two of them to get badly worn.

Unless the chain is actually rubbing on the front mech cage, it won’t be related to the front mech.

Easy first test: can you see daylight under the chain when on the big ring?