I started another thread on this, but I had the description of the problem quite misleading and can't edit.
Basically, the problem lies within the motor casing and I don't know if it's terminal or not.
The motor seems to be operating fine - but there is a higher pitch to the noise than on a comparable DC07.
The suction is much weaker than it should be - if I block everything up, the pressure release valve doesn't open (it's not sticking - it's just not enough vacuum).
Yet, the motor is pumping out far MORE air than a comparable model - it's like a jet engine around the base of the cyclone drum.
I'm guessing that the motor is pulling in most of this air via a route that doesn't involve the pre-motor filter which is why the suction is low.
I haven't dismantled the motor container yet - just wanted to know what I should be looking for inside, or if it's a common terminal fault, then I won't bother.