I spoke to someone in their complaints team today. I nice honest chap called Gordon, and he confirmed that the link from my exchange (I forget the technical team he used), is flagged as Red (they scale from Green, Amber, Red and Black) in terms of capacity/utilisation.
Red means some congestion, but not enough to address, but black means they'll think about it, but that could well be 6+ months before the issue is resolved, apparently (planning, etc).
I've been given the option of leaving without a penalty, or sticking with the service receiving a discount. The vf shop is town is flogging the up to 38Mb service now at £20/month, so I can only imagine the service will get worse, and from the sounds of it fixing it isn't a fast process. Thinking I'm best off jump ship, but most likely I'll have to pay an activation fee as I'll need an engineer to make a physical change in the exchange for my line to move me off the vf equipment.