Well to date, the noise has gotten a little louder, but nothing has exploded or fallen off yet.
I have replaced all 4 tyres and that made no difference whatsoever.
I have taken the car to Chester Automatic Transmissions who got the car on the ramps, dipped the oil in all of the diffs and took it for two separate test drives and they are convinced that it is
not gearbox/diff/prop shaft noise. In fact they said for the mileage, the car drove very smoothly and quietly apart from this overrun noise. They were perplexed and said it was something they had never heard before.
The noise appears to come from around the steering column area and is purely road speed dependant and not engine speed dependant. They did not discount that it could be something like a CV joint or wheel bearing but said that it would be noisier going round corners/tight turns, which it isn't.
Sometime this coming week I plan to get the car up on the jack and test the wheel bearings myself. This is the only thing I can think of. Unless anyone else has any more ideas.....