Hadn't expected this thread to blow up quite as much as it did, but clearly I wasn't the only one having this issue. It's taken a while but finally I've gotten around to trying one of the proposed...

@David-TP thank you, I had an email from one of the support team on Monday suggesting I try reassigning one of the satellite Decos as my Main Deco, which did not solve the issue although it did...

I've upgraded from Deco M4R to PX50 as I wanted to move to WiFi 6 but also utilise Powerline as a backhaul for extending my wired network to a corner of the house where there is no existing wired...