If I use my phone and 3G through this the loading problems for think broadband and Facebook vanish (UK Mail also becomes accessible). Now correct me if I'm wrong but this leads me to believe talk talk have purchased adequate capacity from BT wholesale at an exchange issue (as I don't have speed issues and using 3G through the signal booster all is well). So does this mean that the problems lie inside the talk talk network?
The fact that you get zippy data on the mobile implies that the data throughput on the VPN (from the Home Signal unit back to Three) is working just fine, and it shows that the connection from the Three proxy out to ukmail is working fine too.
If the VPN runs fine, then the underlying throughput of data from mobile to TalkTalk (via the FTTC link to exchange, and exchange into TalkTalk), and then from TalkTalk to Three, must be working just fine.
However, the problem you experience on the PC shows there is a problem in the path from you to TalkTalk, and then from TalkTalk out to that particular destination.
The central part of those two connections (from router, via modem, to exchange, then on to TalkTalk) are identical - and one test shows it is working fine.
Therefore your culprit is highly likely to be either be the PC itself, or the way your PC talks to the router (WiFi?), or in the link between TalkTalk and ukmail.