Well I got it fixed last week, £200 - Full service, hinge replacement, new chain, new sprocket, pads and a hinge. All was well with the Brompton until Tuesday night. When the slow grinding noise came back on my ride home. Turns out it was the pedal after all, that was making the noise.
So a full 'Gold' Evans service does not include taking the pedals off and re-greasing them? Any way I have done it myself now and no more noises, yet.