Many thanks for the replies.
I will try rotating the wheels to see if that helps. When I bought the car it had the standard fit Yokohama tyres which I hated. Road rumble and grip were appalling. 18 months ago I sourced some Goodyear Eagle F1 asymmetric 2 as they were rated best for wet weather (in Cumbria a must!) and had them fitted by Lexus Carlisle. The difference, I think, is incredible with greatly reduced road noise and much improved rided and handling.
I have owned this car for 4 years now and in that time Lexus Carlisle have done all the servicing, warranty and mot work on it. I don't think it is the rear brake calipers as one was replaced at last year's service and the other, along with a front one, was replaced in the 2013 service. However, I will get them checked. Lexus too initially said it may be a bearing but it appears that is not the case.
The noise reminds me of the time when as a child you stuck a piece of cardboard in your bike wheel spokes a with the resulting clacking sound when they went round but this is more like a dull thud, like I'm driving over something. It just started one morning, I got in the car and thought 'what's that noise' as I accelerated up the road. The house next door had some roofing work done at the time and I had found some roofing nails on my drive so I naturally assumed one of them was embedded in the tyre but thats not it.
Once again, many thanks for your replies.
Chris