It is illegal to notify gaswork as your own that is done by diy enthusiast. The reason this is illegal is that diy-ers are not competent to do gaswork. The only people that will do the job in this way are cowboys.
But the fact remains, as has been discussed here many, many times, it is perfectly legal for the OP to do his own gas work - subject to the usual provisos.
He can of course, also get the work notified - as has also been hashed to death here previously.
I am not commenting on the advisability of doing so though. Personally, I no longer fit open flued appliances at all.
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