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 :-(