Hi Giles, I think you refer to the signal strength readings which are expressed in dB as a negative figure. You should be looking for the lowest figure e.g. -75dB is better than -85dB.
Once you select it, you have to click on "change" then "apply" and the system will reboot.
After about a minute you should then try to connect to any website e.g. BBC News, the access point will then re-direct you to a login page where you have to enter your username and password for the BT or FON system. I note that when logging in to BT there are several options, e.g. as a Broadband customer or BT WiFi etc so you have to select the right one.
They recommend that you should have -80dB or better, but I have successfully streamed live TV on a signal strength of -86dB; and in the last couple of days was able to use FaceTime using an access point over 2 miles away; so it is a great system.
The people at Motorhome Wifi are very helpful and when I have emailed them the reply has been back in a couple of minutes, even out of hours; so if the problem persists contact them as the first resort.
Hope this helps.