wrote:
After I take a shower (mixer),i start to hear this hammering/air lock
sound from my pipes.The bathroom is on the first flow so is my feed
through electric geaser,with a header tank in the loft.it doesn't go
away until I open the bath cold water tape.The water spatters out then
i get a steady flow after which its all back to normal.What could it be?

This might be water hammer, which is often caused by a worn washer/valve on
a cold water storage tank or toilet cistern inlet valve.

http://www.diydoctor.org.uk/projects/noisypipes.htm
http://www.diydoctor.org.uk/projects...nks%20etc..htm

A new washer or valve usually cures the problem.