Finally got the car in to get the knackered back box sorted out.. The old power flow one was fooked. Apart from the flange just about falling off, the seam on the box was goosed, so ended up with a nice new shiny one. The guy who did it used to do the power flow ones where I got mine said they used to use rubbish and the seams always went.
The new one's a bit bigger and matches the rest of the system, and has a bit more of a fruity sound to it, and a bit of a burble coming off the revs low down. Pretty happy with what they've done.
Seems a bit freer at higher revs, so wonder if the back box being goosed was interfering with the flow.