I can easily get over 100Mbps over wifi on my Homehub 5.
There is no reason for wifi to start off slow and increase over a few days. The BT helpline often say things like this to get you off the phone. Not that it's any good ringing them anyway, you know more than they do.
You have FTTC so your Homehub 5 is correctly plugged into the master socket.
To find your line stats, you can go to your Homehub 5 web interface by browsing to http://192.168.1.254
, then select Troubleshooting, then select Helpdesk. You should see your line stats but you may have to logon using the password on the card on the back of the Homehub first.