Well I managed find to do all the checks and check all the settings, took me longer to get around to it than I though it would.
All the devices have the correct settings. So that can be ruled out I believe.I've manually assigned wifi channels and let the HH do it and there are no benefits it terms of stability either way.
The problem seems to be down to the number od devices using wifi at any one point. Once "critical mass" anything over that causes my sons connection to die. In terms of "critical mass" the number seems to be 4-5. It's not like a switch it seems as if it tries to keep up and then decides sod it, he's furthest away (upstairs) he can [censored] off. If all the devices are in the same room it just refuses to connect the last one. As soon as you switch off any device it kicks in again and tries to reconnect any other device seeking a wifi signal. I suspect it would also depend on a device wanting to access the actual internet rather than just be connected. You can be connected just have zero internet access.
It sounds like a dodgy HH to me but maybe they are just rubbish and always like this, I've only ever had one. All my other wireless routers have worked fine with multiple wired and wireless devices connected and functioning.
I'm tempted to report it but wonder if there's any point. I've dealt with BT Customer Service before and I mostly end up wanting to kill people.
I can't really move the HH from where it is and although I could run a cable at a pinch, I feel disinclined to do so.
As it is my son is about the only computer connected to wifi no, my wife is using a cable, I already was and my mobile phones use it occasionally.
I hate wifi, you know where you are with a [censored] wire.
Thanks for reading and all the suggestions and help.