Have taken the car to an independent dealer who suggested the brake booster is on it's way out (I could tell my the hissing my the brake pedal in the footwell).
I also came across this video as the car was fairly noisy -
https://www.youtube.com/watch?v=QHLyubTrIQ8
He talks about checking various places for leaks in the system, could the brake booster be one of these leaks or do I need to go hunting for another?