I have recently had a new boiler fitted in the attic by a Corgi gas fitter. In order to get the gas supply to the attic he ran a copper gas pipe up the outside of the house to the attic. A friends son, who is also a Corgi man, then told me the pipe should be lagged to avoid splitting. So, should an outside copper gas pipe be lagged or not?

There is nothing wrong with exteral gas pipework and it does not need to be lagged, why would it split, gas does not freeze, I'm sorry to say your friends son does not know what he is talking about.

Without consulting the bible for correct phrasing, the rules go something like this....suitably protected against the elements and physical damage, clipped at appropriate intervals, correctly sleeved where it exits/enters the fabric of the building. Someone else may offer more but there's no specific reference to lagging as such.