" but nevertheless it hasnt been fitted by a gas engineer then you cant use "
I'm sure that GAS ENGINEERS have better things to do than install pipes,
If the pipe in question can be checked along its length for any branches/valves ect, and also may be flushed out (if required) I cant see a problem.
You wouldn't hesitate to use a cold water pipe to carry hot water.
Providing the pipework is chesked whats the problem.
thats what i said, but can you GUARANTEE on the retention of your gas registration that all is 100% correct with every fitting and every jointing material used? if yes, go ahead, if not then you cant use it