Thanks for the advice on this. There's probably not much more I can do in my house to improve the connection. I gained a 10mb increase in speed last year by relocating the modem to the master socket nearest to where the phone line enters the house, and disconnecting all other extensions and wiring.
I'd hoped my stats would've been good enough now to go back on fastpath after two years of the internal telephone wiring causing such a large speed loss, and hence DLM intervention. I thought perhaps that DLM had got stuck, as I'd read that can happen.
I'd be hesitant to raise a fault with BT on this, as in all likelihood they'll say there isn't one, and in fairness they're probably right!