ahh ok

may be this one for me then ?

No, the sealey unit is marketed to the use of finding leaks in vehicles, around windscreens and various seals. The transmitter creates the frequency for the receiver to pick up as their is no other source

Will be getting one .

The sealey needs the transmitter to work doesn't it ? 

No, I haven't used the transmitter.

Dunno, give them a ring and see... they are quite approachable and sometimes have ex-demo stuff

I have been using the accutrack leak detectors for years and they work great.

http://superiorsignal.com/products/ultrasonic-instruments/accutrak-vpe-ultrasonic-leak-detector

I have a sealey one designed for car windscreens, has helped me out a few times. Not something i tend to use every day, but its a god send when you cant find the leak easily

yes and how much does it cost?

here:
http://www.vacinnovation.co.uk/page/1sntf/Vacuum_Connectors_Hoses_and_Gauges/Ultrasonic_Leak_Detector.html