In short, it was no better or maybe slightly worse for stability on the wireless. The worst point was when we were both in the lounge opposite the router with laptops and the SSID just dropped (dropped as in it vanished entirely from the list of available SSIDs and then came back) for both of us at the same time.
I've given up and going down another path, I've installed an access point from Ubiquiti and disabled wireless on the HG633.
So far, touching everything wooden this is providing a solid connection to the TalkTalk router and on the 5GHz I'm getting full speed.
I'll be sending one of the HG633s back shortly.
It's ok, it's pretty much as I'd concluded at the start the HG633 just can't handle the number of devices (not all that many ~20) or that coupled with the noise the TalkTalk IPTV service causes.
I've been working from home today on the new wireless AP and it's been trouble free.
Hopefully TT start giving out better routers in the future, but now I'll not be needing it.