Thanks all for your responses. Ive dug around and found the first section of pipe from the bottle to what I now assume is the non return vale looks dirty. Beyond this point the pipe appears to be clean/looks clear, so I'm hopeful the valve is the issue. I've also removed the outlet from the the scuttle and it seems fine. So, am I best cleaning or replacing the valve? If I'm best replacing what do I order and where from? Thanks, Andy