A couple of questions regarding extending gas pipework:

1) It is illegal to run gas pipe in a cavity wall because of the possibility
of gas build-up, does this mean it is preferable where I have the choice to
run gas pipe along a wall rather than under floorboards?

2) How do I clear the existing pipe of gas before soldering the new pipe on?
The pipe comes down from the meter, under the floor (presumably horizontally
but I've got no way of telling) and then up in a cupboard which is where I
will be making the join. So I would imagine most gas will escape once I open
up the old pipe - but with things like this it is best to make sure! Should
I open up the test nipple at the meter to aid dispersion? I don't have any
gas analysis devices.
N.B. I am fully aware of the need to purge air from the new installation.


Thanks

Pete.