I have answered my own question to a certain extent by interent searches, which also turned up a thread on this site, see below.
I further located the source of the noise to where the rod connected to the brake pedal passes through the bulkhead to the servo, it's easy to hear when your head is in footwell!
Have found 2 other people with same problem.
From the thread below it seems a new servo unit is required at a cost of near £600 fitted by Ford.
So I'm not impressed at having to replace this on a 5 yr old car with only 38,000 miles on clock and always serviced by Bristol Street Ford.
Anyone had any success at getting Ford to contribute to cost of repairs in this type of situation, as feel this must be poor design or bad batch of parts and not fair ware and tear?
The servo does not effect the brakes directly - the brakes will still work as normal, but will require increased pedal pressure to slow down or stop the car, if you are managing to do this without having to use both feet and herculean strength, its probably ok to still drive the car, it may deteriorate though
in my opinion (i normally comment on "technical/ mechanical" - not things like "warrantees/ insurance/ political" etc) - If the car is outside warrantee, it is unlikely that Ford will contribute the cost of anything, everything wears out/ breaks (not brakes :lol: ) on these cars, there must be 1000 design flaws, at least (some would argue there is 100 times that) - it depends on your perspective,
I seem to have cured the hissing noise (caused by air being sucked into the servo without the brake pedal being operated).
It is normal for air (atmospheric pressure) to pass into the servo when the brakes are operated, in this car's case through an air filter encased in a nylon tube surrounding the brake pedal push rod, but there is either a seal or valve to stop this happening when the pedal isn't operated. For some reason, possibly dust/gunge, this seal was not working 100% allowing air to be sucked into the low pressure of the servo all the time, but not enough to effect its operation noticeably.
The cure was with the engine running I sprayed WD40 in the area around the brake pedal push rod air filter, so that it was sucked into the seal or valve that was leaking, then I pressed the brake pedal down, but let my foot slip off the end of the pedal so that the brake pedal pinged back against the back stop (its rest position) several times.
I think the shock/vibration of the pedal hitting the back stop (caused by the return spring) may have dislodged dirt or gunge.
Result, no hissing for over a week now and the car passed MOT 2 days ago no problem. Hope this helps someone else as the the other chap who I contacted with an identical problem had his servo replaced by a garage at a cost of nearly £600 :-(