Are you on a "Retransmission High" profile? Ask Plusnet to give you the results of a FTTC GEA service test, which shows the profile details.
If you are on a Retransmission High profile, then the IP Profile is a lower percentage of the sync speed, by design. Actually the GEA test will report a lower speed than the "sync speed", and the IP profile will be 96.8% of the lower speed, but again, I think this is by design, and not a problem with the system.