Mike the switch I selected was the GEMS
PS61-19-4MGZ-B-SP
http://uk.rs-online.com/web/p/pressu...tches/7794238/
They are cheaper than buying a cylinder but are heavy. They call them subminiature, hardly a term I would use! I have checked it with the pressure gauge and adjusted the switch point. Seems to work well. Unfortunately the BSP converter to 4mm push fit had the wrong thread on it so I very careful drilled out the hole to 4.2mm and tapped M5. I drilled and tapped it horizontally so there was little chance of bits dropping into it, and made sure to tap it on the bench first before having a look. My test was with the continuity test meter but I have no reason to think it will not work substituting it for the cylinder and micro switch.
John