1.8 406 injection system leak??

[petespeugeot]

Hello
whilst servicing my father in laws year 2000 406 1.8 petrol estate last weekend I noticed a curious problem.
With no keys in the ignition about every 4 or 5 minutes there would be a sound like a relay clicking followed shortly by what sounds like a small hiss of air. The click is coming from the left hand side (drivers side, uk) and the hiss would seem to be up near the injectors.
There are no apparent signs of fuel leaking nor any excess smell of petrol.
I wondered if this could be due to a pressure regulator failing on the injector system.
The idle is becoming a little iratic and it is not running great even after a full service.
It has done about 100k and been well looked after, and to now has been a good car. It has also been difficult to start occasionally if left for a few days.

Any suggestions and advice would be most appreciated.

Cheers, Jason