I've a bristan 1901 shower feed via a showermate 1.8 bar twin pump (not an electric shower). Had a guy from Bristan fit a new cartridge a couple og weeks ago. Since then the pump was on occasion, not coming on. Now it's completely gone. I'm pretty sure the system is airlocked, but can't seem to fix it myself.........any advice on how to do this?? Guy from bristan is gone to ground and I can't contact him.