What are the regs with regard boxing in a gas pipe?

[doogyscoot]

I need to fit a shower to a wall which currently has a gas pipe running down it so the wall will probably be boxed out for pipe work, it is a straight run of pipe with no joints but I a little confused about the regs regarding boxing it in. I have been told that in some cases the pipe must be vented top and bottom but in other cases if the pipe has timber battens fitted along its sides to cover its height this is unnecessary? Any advice appreciated.