Hi, I was given a £120 diagnostic fee and whatever the cost of repairs, using the description given in Simons post I ordered the part from a dealer $75 (inv VAT), I have waited 2 weeks before reporting the results in case something didn't work after I replaced the sensor.
I pumped the brake to equalise the pressure, clean the area from dust and dirt (seemed the right thing to do as I was going to expose the servo pressure mounting to the outside), unclipped the electric cable, used a small flat end screwdriver to twist the clips over the small lugs you can see in Simons photo (bottom left), as I was doing this I applied pressure by pulling the sensor away from its mounting and it came away with ease. To replace just line up the sensor apply pressure and it will click into place, attach electric cable.
Both warnings have gone and all feels and looks serviceable. Hope this helps others, Thanks Simon.