yes the noise is intermittent not a thudding but like metal grating on metal
what in your opinion causes the locking of the brakes when the car has been left sitting overnight?
I left the car in to ford for a cleaning as you suggested and it turned out that the brake pads were worn on the inside and had to be replaced. The mechanic hadn't noticed this a month ago when I'd taken the car in with the problem. The grating sound has gone and the brakes behave normally when the car is driving but in the morning there is still the locking of the brakes with a big bang as they are released by the engine.
To be fair they gave me a reduction when they had to fit front brake pads on the second visit. £80. Did they check back brakes? They said so but there is still a vestige of metal on metal sound when i brake so my guess is no. Think I'll go to another garage since from what you say about the front brakes this was a pretty basic mistake.