@David-TP

Yes, It was exactly like that. Surprised I didn't come across it in my searching.

Funny thing is though, I'd had the Extender in working fine for a couple of months before I started getting the problem, so I never associated it with the Extender. I actually contacted Talktalk, my ISP, and they saw problems and unbenown to me capped my speed at 2Mbps. I had 2 engineers out to try and fix the speed problem before being told they had capped it. That was sorted so only then did I start to suspect devices (>30) on my LAN.

Perhaps the Extender auto updated its Firmware?

Anyway, yesterday I set the Extender DHCP to Manual, giving it a fixed IP address (like all of my other fixed devices) and the DHCP Server setting to Off. I have not experienced the problems since (only 24Hrs I know).

I'd still like to know what the DHCP Server setting actually does.

I'll add a post on the other thread regarding my experience.