Thanks for your reply.
As the line tests are not detecting any faults then we can request an Openreach engineer visit but we can't guarantee that the engineer will increase the speed any further if it's within the predicted range.
Possible engineer charges may be applied if the engineer is unable to detect any faults and advises that the speed is ok.
An update to what has happened. I gave up with TalkTalk for now, have switched to Vodafone (as they guarantee min 55mbps or 15% discount) and without any engineer visits the current sync speed is 57mbps.
Sorry to see you go, enjoy your new ISP