This site http://loctite.com.au/int_henkel/loc...0439screen.pdf (page 8) makes it sound like the right stuff for the job... I may give it a go.
Loctite 577
Recommended for all coarse metal threads. Suitable for applications at low temperatures requiring fast cure, e.g. outdoor plant maintenance.
Sydney Water Board Approval Number 95/00043.
Gas; Approved by AGA. Approval Number 4787 to a maximum pressure rating of 2600kPa.
WRC approval for use with potable water.