Thanks to everyone I am pretty ignorant on this, but assumed its
kbtus. Its probably 30 odd years old. The 15mm supply has never been a
problem in any way, but thats bureaucracy for you.

If it has worked perfectly for 30 years, then leave it, unless it has failed
an inspection. Without doing the calcs, and assuming few elbows, the run is
probably just below spec. Perhaps someone will be kind enough to do the
calcs for you!

Christian.