I think (touch wood) I may have found the noise...
I took the rear wheels off and inspected the brakes - they were fine.
Checked the bearings - also fine.
However, due to the high winds tonight (luckily) I could actually hear the noise with the car stationary...I managed to trace the noise to the rear beam (not actually sure what it's for but it's in front of the back wheels and goes across the width of the car).
At each end of this beam is a large hole. I blew into them, which made the EXACT noise that's been sending me insane over the past couple of weeks...
So, for a quick fix to see if that's the source, I wrapped the beam at each end in a cloth and gaffer taped it tight.
Went out for a drive (again in high winds) and the noise was not present!
I'm not getting my hopes up because I've attempted to get rid of this noise so many times without success that I'm completely pessimistic about it....
Only time will tell
While on the subject of noises, something seems to have come loose behind the steering wheel and rattles over bumps... any thoughts?