Well, it took a while and some tense phone calls but an engineer came out today and performed a health check. Tests at the drop cable were much better than tests at the master so he replaced the wiring between the two and hey presto we're back up to 7+Mbps with a healthy looking margin.

Bit more pleased now after a very frustrating call at the weekend where I was told 4 was all that was possible and was capped at that!