Just an update on this. I think this is tyre noise as the sound seems to be subsiding. It may just be the particular point in the lifetime of the tyre was creating the noise. 

Final one on this in case anyone was following.

 

I noticed "stepping" on the front tyres a month ago. Got the front wheels swapped left to right and got the tracking done.

 

The sound has more or less gone away now.

Hi all,

After all that, it was related to either the front shocks or springs. I’d convinced myself that the sound had got better from swapping the tyres front to back but the sound came back and I just accepted it.

I got new front shocks and springs on Friday and the noise has totally gone as has a considerable amount of road noise! The difference is very noticeable.

Took a while to get to the bottom of it but suspect it was the shocks resulting in excessive vibration at certain speeds.