The council has sent their finest man to sort the problem and he's changed the pump. Hasn't made a blind bit of difference, so now they're going to flush the system. It looks like a cold winter ahead!
Thank you for your input The Builder, I really do need to consult a professional.
gucciman, you might be right. I have a large brown stain on my kitchen ceiling from the mucky water in the old tank that the plumbing cowboys failed to remove.