In message ews.net,
Doctor Drivel writes

N.B. I am fully aware of the need to purge air from the new installation.

As always if you need to ask you probably shouldn't be doing it...

1) Through cavity is fine but there are extra rules. See BS 6891

2) No need to purge gas from the pipes. You will need to seal the meter
outlet if working within 2m of the meter. The gas can only burn if comes
into contact with air. Even when unsoldering fittings the most you even
get is a small flame which goes out in a few seconds.

I have seen a flash dow 6 metres of pipe and a boom as the guts of the
meter blew.

I bet you have

--
geoff