Sounds as if it may be wired incorrectly. Have a look at
http://www.diyfaq.org.uk/plumbing/pl...ml#midposition which may
shed some light on how it is supposed to work and on how it should be
wired.

Thanks for the link - I was already suspecting that it was wired
incorrectly, quite how I sort out the mass of wiring Im not sure - looks
like a long test with the meter!

Any other suggestions, whilst I keep on reading?