Except your attitude, I cannot remember any post getting feedback such as this, considering the question, normally it would have been dismissed out of hand, NOW you`re helpin` dudes! Good on ya most clever Chris dude......
I have gone through the Regs and I cannot find any definitive wording stating whether or not you can or cannot do this mate.
Mind you thats pretty usual in the regs or as CORGI say "mmm thats what we call a grey area"
My tuppence worth is if you cannot see all of it then don't use it for the reasons you have already pointed out.
I have used a redundant water pipe for gas before, I could see every inch of it and whack a compressor hose on it for a while to dry it out and Bob was most defiantly my uncle.
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