Basically, the excise duty only becomes payable when the whisky leaves the bonded warehouse.
The deal was that they buy the cask back for £1000 plus 36 bottles, which I negotiated up to 48, 24 now and 24 in 5 years time. The bottling costs and excise duty on the bottles I get are covered by the distillery as part of the deal.
If I hadn't have wanted to sell the cask back I would have had to pay the bottling costs and excise duty up front at bottling time, which, if I remember correctly, was about £5 and £15 per bottle respectively, so the 260 bottles they estimated I would get from the cask would have cost me approx £5200 plus the original £895.
As I had no way of retailing 260 bottles, I took them up on their offer.