Hi and welcome.
As Eric has already said, unfortunately youve been told incorrect information. The stats from your router clearly show that you are quite capable of the higher speeds
I would also recommend you do a BT Performance test to check your IPprofile.
http://www.kitz.co.uk/adsl/btwperformancetest.htmIm a bit surprised that Plusnet told you this as normally their staff should know better. In fact the Plusnet profile is meant to automatically follow the BT IP profile, but it looks like something may have gone wrong.
When you have your IPprofile, get back in touch with PN, informing them that your sync speed is 'x' kbps and your IP profile is 'y' kbps, but your Plusnet profile seems to be stuck at 7.1 Mbps and can they please amend it.
If you still have problems, let us know and someone here will give one of the PN reps a prod.