Upon examination there was a visible rubbed area on the exhaust and bar about a centimetre long.
The two back box exhaust hangers looked chunky but were still allowing the exhaust to move around quite a bit so I decided to replace the inner one (nearest the centre of the car) with a firmer Powerflex polyurethane hanger (part number EXH005) which had the same distance between the hanger holes as the ones fitted.
This had virtually NO effect as the Powerflex hanger was just as flexible as the chunky hangers I had on already!
I then bought a pair of slightly smaller Powerflex hangers (part number EXH015) and replaced both original hangers and these tightened everything up nicely. The back box was pulled up and away from the anti roll bar.
Hope this helps if you have the same problem.