I've just moved the box to a different wall in the same utility/boiler room - which is technically outside - that's the problem, a third of the house, the left edge (kitchen, utility room, other bit), is a single storey extension and so has thick walls between itself and the main area where the ceiling WiFi AP is. Also being single story, I'm a bit stuck for throwing CAT5 or other cables over the ceiling - there's no rooms above with floorboards that I could lift, and of course plastic trunking isn't wanted in a house. I'm going to have to pull apart a plasterboard ceiling and make good afterwards if I want to run cables. Going around the outside of the house doesn't seem straightforward either. If I could fit it where the existing electro-mechanical thermostat is fitted, that would be a dream, but alas they have run 4-core cable and I need 5 cores.
Anyway I've moved it to another wall in the utility room for testing. This is a location I tried previously too, and I now see -57dBm overall signal strength reported on my access point, and a signal to noise of 50 to 52dB. This is a drastic improvement on -76dBm ~ -79dBm with an snr of 30dB that I started out with!
The true test however, with this Drayton Wiser Hub R, is to send an HTTP request to it. Pings are usually fine, but if you send an HTTP request (just put the IP into a web browser, which should return a HTTP 401 Error Unauthorized', and keep clicking refresh, how many times you can click refresh without it hanging and resetting the connection seems to be the true test of whether it's going to work properly or not, i.e. whether the App on your phone is going to sit there and hang for 5 minutes saying that it's "Searching for your Hub..."
I am still having this problem though, so while the WiFi numbers reported on my AP are better, it's still not reliable enough. It is much better, and I suspect I will be able to find a reliable location inside the utility room now with the improved antenna. I am going to do as I said above and fit the antenna internally, then find the best location in this room. If, even with the improved WiFi, it's still not good enough, then I'll put another small AP in the utility room, probably running off a HomePlug Powerline adapter for now, until I feel ready to rip the ceilings apart.
Incidentally, I have had it sat in the livingroom (same room as WiFi AP) for a few days and don't have the problem there.
I think there is some sort of design flaw in this hub overall though, perhaps in its HTTP server. For it to respond to pings just fine, but then the HTTP request hangs, and one hang causes the whole HTTP side to die for a minute or more at a time, all the while it's responding to pings just fine, makes me think that the HTTP server can only handle one connection at a time or something, and has a long timeout. It seems to be very unresilient anyway.