I have read the recent FAQ on this and have closed the locknut. The other end has a Honeywell TRV, http://www.plumbnation.co.uk/site/h...ostatic-radiator-valves-/?activeTab=products, and it is advised to take the head off and fit a decorator's cap. I presume the head comes off by undoing the ribbed metal circle but what is a decorator's cap and is it essential to fit one or does is the valve normally airtight anyway if fully closed before removal of head ? I can't find a reference to decorator's cap on any supplies site.