Low pressure light just started coming on?

Our shower was fine until about a week ago when it started playing up. The low pressure light would come on and then the water pressure would get less and less until it was more of a dribble. It didn't happen everytime but is getting more frequent.Sometimes it would run okay for a few minutes before the light came on.

Can anyone tell me what might be causing it, we have checked the waterpipes in the loft and there is no obvious leakage.

(Mira Sport shower, no other details known as it came with the house)

Megan
May 2009