On Fri, 30 Jun 2017 10:47:05 +0100, "Jim GM4DHJ ..."
wrote:

[snip]

every traditional four storey tenement in scotland had the gas supply in the
common stair .....worked fine for years but then again closes were open
front and back ....

As it happens, I live in such a building and the gas mains in the area
are being replaced at present. I had a meeting with the SGN project
engineer on Wednesday. The riser in the close is made of steel and is
to remain. The cross pipes are made of copper and apparently require
to be replaced if the gas supply is interrupted but this is not needed
if it is not.

The project engineer explained that a surface mounted pipe is regarded
as safer as it reduces the risk of any escaping gas entering a void.

We have doors front and back but I think the assumption is that that
there is still plenty of ventilation. Why would a pipe in the close
present a greater risk than a pipe within an individual flat anyway?