I managed to get it working in the end by disabling the 5Ghz band on my wifi and then going to the Broadlink App → General Wi-Fi Device → Smart setup Mode and following the steps from there.
Firmware updates - yes or no?
I have some of these Screwfix sockets, purchased last year. HA calls them AHC/U-01 with Firmware 46043
Is it worth updating firmware? I see mention of a 57053
My devices work well, just jabber a bit too much attempting to phone home to Broadlink every few minutes. Had to block 'em at the Firewall.
I know it’s years later but thought I’d add my thanks for this advice. Been trying for ages to sort this out on & off, no idea why I didn’t find this post earlier.\
So glad I found this post I’ve been trying to get my BG sockets working for some time.
After following lots of the suggestions, I eventually got a socket working using the BG Home app and the AP process. Once in Home Assistant it played perfectly, just need to get my other ones working now! 
I have two of the BG plug adapters which are labelled AHC/U and had been using them with Google Home. They are discovered by the Broadlink integration but cannot be properly setup. I did get them working in HA by using the Broadlink app even though they were not added to that app. However after a software update to my HA Green they needed setting up again. Having wasted many hours on this, not helped by the fact that the Broadlink app will not proceed if your phone is connected to 5GHz WiFi, I decided to give up and use them for use on special occasions in Google Home.