I see a lot of listings for specialist S/A produce stores in London. Has anybody found one that stocks NTSU or other S/A snuffs for that matter?

Yes, in Wimbledon. Site is http://www.downsouthgroup.com/sa/ and they charge about £1.00 ($1.50) each and will post to UK only. They stock NTSU (the normal version. Black container/yellow lid) and menthol (tastes like spearmint !).This is where I get my NTSU from. Nice and fresh. Straight off the ship. I collect in person when I fancy a trip on the tube. When you go to their web site, enter snuff in the search box.