Engine idle high (1250) but reduces when brake pedal pressed

I have a Focus Mk2 2011, 1.6TDi. 110k miles.  Engine malfunction warning came on and traction control warning light illuninated.  Since then the engine has idled at 1250 rpm. Mechanic advised a Auto-Electrician needed to run full diagnostics.  This was done and the advice was that the EGR Valve appeared to have stuck.  So EGR removed, completely coked and beyond cleaning so new EGR fitted. No change to revs or problem. Codes cleared at the time. Was then advised then to replace MAF sensor, done - no change. Since then Inlet manifold temperature sensor replaced - no change. Manifold pressure sensor replaced - no change. So I'm sitting in the car scratching my head with the engine idling away at 1250 rpm and I happend to depress the brake pedal and as soon as I did this the revs dropped nicely down to about 800rpm. ON releasing the brke pedal the revs immediatley climbed back to 1250rpm.  As a result of this strange behaviour I summised that it must be something to do with the vacumn system. I therefore replaced the PCT valve. No change.  I have repeatedly tried to clear the ECU memory with the old trick of disconnection of battery etc. Anyone come across this situation before? any help would be much appreciated.