I have been in the exact same situation as yourself both with SkyBB speed estimation at my parents house of 14Mbps but only able to achieve 3.5Mbps and when I decided to jump ship in my property from Sky over to Plusnet FTTC I was initially on the incorrect IP Profile, which I believe has been discussed widely in this forum and other places on the web.
Anyway I know its not exactly the same situation as yourself but I can relate.
I would take the advise and grab a HG612 and unlock that sucker.. Do some stats plotting as MrSaffron said you may be able to see a distinct flat line speed test result and this will point you to a possible cap on your product package.
It maybe, worth asking for confirmation
of you IP profile from TTalk. I don't really know how open your Service provider will be on this type of information. Just to be sure you are not on the 40/10 package as opposed to the 80/20 package..
But definitely try to lay your hands on the HG612 modem. You can also do your tests in Bridged Ethernet mode and bypass the router/Gateway to see if you get different speed results just by using the modem itself.
Hope this helps.