Is it too late to plant peas?

A mangetout pod is a developed pea pod! The pod is developing as it emerges from the flower.

I get what you are saying, but I was just repeating what the RHS website says i.e.  "...mange-tout types of pea, where the pods are eaten before the seeds develop, should not become infested." and "After hatching, the caterpillars bore into the developing pea pods and begin feeding on the seeds."  Both of which, to me at least, suggest that the pea moth does not normally enter the pea pod at the mangetout stage.

Yes.

The peas I planted back in July are doing fab.  The Mangtouts are the best I ever had and there isn't a sign of mildew.

If they do grow you might be able to harvest fresh frozen peas in December! 😊