You can get them from http://www.macdee-systems.com/
Although the Metro has been discontinued, the replacement units have not changed the design, and you can buy them direct still. I've had other problems with the same unit, but found that simply turning over my diaphram (so the water flows are effectively reversed) made a huge improvement.
Rohan
January 2010
yes
chris saggers
January 2010
I can answer this question...
Want diaphram for Derwent Macdee Metro 3 part syphon ranbge?