Sorry Wayne, still haven't fixed it ! I am reliably informed that the tube is likely to be a bayonet style fitting, like a light bulb, but I haven't dared apply sufficient force to make it come to pieces.
I'm also reliably informed that it will be the washer - new ones are as rare as hen's teeth, but apparently it can often be cured by turning the washer upside-down - if only one could get at it !
Fred
May 2009