I swapped the tyres front to rear at the weekend and this made no difference. Pressures are correct and tyres are nearly new. This leads me to think that it must be the front wheel bearings or possibly the centre driveshaft bearing and support. If it is a wheel bearing I would have expected the noise to change substantially when turning left or right and it doesn't?
Noise is most noticable at lower speeds (<40 mph) but this could just be because engine and wind noise drowns out noise at higher speeds.