In the North West of England Boots the Chemists seem to be presently buying out all the local independent chemists, which was agreat boon to me as I've just got my hands on two 2.5L and two 1L Winchester bottles for nothing as part of a clear out/refit. Worth keeping your eyes open!
William Ralph Dean - Inspiration to four generations of my family
It looks as though the 2.5 litre size Winchester bottles are the largest that are commonly available. The 5 litre size demijohns might be suitable, provided they can be made air tight. I might also check around the local chemist stores as suggested.
Thanks to everyone who replied.
When I worked in a pharmacy, we routinely discarded the 2.5 litre 'Winchesters' that in which cough linctus was supplied in bulk, and most pharmacies get through a LOT of cough linctus. You'd be doing them a favour by taking the bottles off their hands.