Thanks every one. I found a knowledgable plumber, who explained to me that a natrual gas valve is lubricated by grease and the diesel oil
would wash all of the grease away. He then got out his catalog and showed me the stop tap for an oil
fired central heating
£3-10p or about $5-00, which includes an automatic shut off in a fire.
I purchased the same from my local plumbers merchants, who charged a little more. The tap is made of brass, has a left hand thread, which means the knob is out for off. The only defect seemed to me was that you could screw the knob right off, so I covered the last two threads with 'dip it, wip it', to make it harder to unscrew the knob completely. Taking the knob home with you would make it harder to steel
the boat though.
Unfortunately going to my nearest chandlery
is a trip of 56 miles and I dont think he stocks them. None of my mail order catalogues has anything.
Now all I have to do is fit it.