Hot & Cold water downstairs is absolutely fine.
Hot water upstairs is also fine. The problem is with the cold water upstairs. If I turn the tap on it runs at a low pressure for about 4 seconds and then just trickles to a stop. The toilet refills incredibly slowly as well.
The cold water tank is in the loft, and is filled to the top with water, so no problems there.
Not sure what else to do.