How can I fix my Aqualisa Aquastream bought in 2007?

Hi it's leaking when i turn it on, water pours down the tiles. A plumber friend who took it apart once before when it did it said that it was a little plastic bit that has broken off from the pump housing. He happened to have a spare in his van that sorted the problem. A year later it's done it again, my friend said that you cannot buy that part separately you have to buy a whole new pump. Surely, there must be a way to stop this stupid bit of plastic breaking in the first place but is it true that I cannot get this housing that goes around the pump?