Booked it into the local garage this week, but took my Dad out for a short drive first. He got the rear passenger footwell and stuck his ear to the floor - he thought it was coming from directly underneath the handbrake - the VCU.
Took it to the garage, told them what I knew it wasn't, and about everything I'd tried, and left it with them. A couple of the lads took it for a drive. Then they got it up in the air and ran it. They (3 of them) were sure it was the IRD or the gearbox.
They started stripping it down this morning - once they dropped the prop they decided to run it again - no noise.
I was right - it was the rear VCU bearing that had started to go. It was fixed by dinner-time !!
The lads were surprised that it was making the noise it did - usually whines, but mine sounded like a helicoptor. I was also beginning to get a vibration, which must have been a slight wobble on the shaft.
So, there you have it.
Cost me £175 - they got the new bearing from Landrover for about £70+VAT, which they said they thought was a bit steep, but it was complete with the rubber bit and everything. Either way it was better than the IRD.
It sounds lovely and quiet now, which is great except I can now hear a hint of a hum coming from the rear offside wheel bearing...... that can wait though.
Thanks to everyone for their input. x