PS I have a replacement PSU on the way so that will be plugged in next time too. I know changing many things at once is not good but if the fault goes away I don't mind too much not knowing what exactly fixed it ;)

"…you could always set the Plusnet router to DMZ directly to the DHCP’d (NAT’d) Draytek router, this has worked for me in the past and will allow VPN through directly, without using the Matcher service."

Many years ago (when I first had VDSL) I tried that with my Plusnet Hub 1 and a Draytek 2820 and couldn't get it to work.

Worth a try however.

My fingers are crossed that the PSU does it!

I agree, it does sound a lot like the PSU from your take on it.

(Although, I swear OP mentioned he changed the PSU in an early post, but maybe I misread that.)

EDIT:
just checked the previous posts and it was actually @clyde123 who mentioned changing his PSU…

New PSU in my hands now, hope to make avisit this weekend to plug it in. I have also thought of inserting a Gigabit switch between the router and ONT (ideally would be a hub but I only have a switch) to see if it is the router or the ONT that negotiates the link down to 10/100M from its initial 1000M - that should work shouldnt it?

As you say, a hub would be better. My guess (and it is only that) is that with a “dumb” switch it would work ok. If it works at all, it won’t be unreliable after a time.

It’s got to be either the Draytek or the ONT. The other thing I wonder about is does either device run hot to the touch?

Getting Openreach (via Plusnet) to replace the ONT would be hard without having the Plusnet router in circuit.

I can only try the switch (its a tp-link simple unmanaged one) and see if connection is successful, if not it comes straight out.
Newest development is that the Draytek has suddenly sprung back into life without manual intervention - only done that once before - and I was able to retrieve the latest log file. Interestingly the last thing that happened before the PPP link startup began was a PADT from the Draytek. I have yet to search the previous log file to see if I can match the session number, but my suspicion is that the Draytek is not sending the PADT until a very long timeout expires and that's why I lose connection for so long. If it sent it immediately I would have these disconnections still but would reconnect immediately and I would neither notice nor care about the disconnect.