Sorry for the confusion. I replaced an old electric shower with new bath taps with a shower mixer outlet. This is the tap
http://www.bathroomheaven.com/emeli...th-shower-mixer-tap-exc-shower-kit-10346.aspx
It says it is suitable for low and high pressure systems, is this not really true?
I don't mind getting a plumber in but don't want to waste time and money if it is something simple or stupid which I can fix. I'm only in the flat 2 nights a week so would have to take time off work to get someone round.
Hold on... have you tried to block (with a finger) your the mixer tap or some other basin tap?
Can you try the following:
- try blocking with your finger the mixer tap - can you block the flow?
- remove the shower hose from the mixer tap and try blocking with your finger the shower hose connection - can you block the flow?