Ok, just been out, in stead of going into town, I went out on a quiet country road, and was able to do 60mph within 2 minutes of starting the engine/leaving my house. I rarely do that, nearly always driving initially on congested roads and rarely getting above 30-40mph.
Interestingly, just now, the sound only suddenly started after about 10 minutes. That suggests that whatever is making this sound has to warm up. So I was driving 60mph-65mph with no sound for 10 minutes.
I got onto the motorway again, tried all the things: -
Turned a bit either way - no change in the sound.
Applied break - no change except the sound lowered in pitch soon afterwards as the break inevitably slowed the car (the sound decreases in pitch with speed decreasing), but it didn't change instantly as I braked, so I don't think its linked.
Gear/depressed clutch - no change in the sound at all as I did so, it just continued the same throughout the gear change/clutch depression, as I maintained 65mph in either 4th or 5th gear. No change in sound at all whilst changing gear.
It makes you think its tyre sound, once they have warmed up.
I don't think its wind whistle as it took 10-15 minutes to happen.
I don't think its brakes or wheel bearings based on what you say about what to listen for.
I don't think its any belts, as the sound would change when changing gear (Is that correct?).
That's me guessing, as a non-mechanical person.
Thx again, so indebted to you both for your help here.