On Sun, 14 Feb 2010 21:50:36 +0000, Robert
wrote:

For the last couple of days the cold water from our taps (direct from
the mains) has been coming out of the tap cloudy. The cloud effect is
caused by tiny bubbles which gradually fizz out leaving the water
clear. There is no odour, colour, or bad taste so I presume its safe
to drink

Perfectly safe.

but where have the bubbles come from and why?

Air. The water in the reservoir is cold and has a lot of dissolved
air. In the pipe it is under pressure so the air remains dissolved.
When it comes out of the tap the pressure immediately falls and the
air comes out of solution and turns back into gas bubbles - exactly
the same as opening a bottle of fizzy pop.