Guys, thanks your responses... I had hoped that 'someone' out there might have done it recently, but seemingly not...
cavortingcheetah
That is indeed the fall-back... I have done that in the past, and I have carried a couple of 1½ litre empty spring water bottles and decanted spirits into those before putting into checked baggage. The PET bottles are remarkable robust, and it avoids the problem a colleague had recently when presented by his hosts with a bottle of Sake as a farewell gift at the airport. The entire contents of his suitcase needed dry cleaning and/or dhobeying, and he says that his suits still smell of it...
ExXB
Thanks for the link... I had spotted that, but it is not specific about Duty Free... although indeed one can argue that if the rules set out for Liquids, Gels and Pastes say "no problem on domestic flights" then presumably they'd apply for a couple of litres of Scotland's finest.
