I fitted a 1/2” bsp s/s tap to my Cygnet with a 1/2”s/s running nipple and washers inside and out sealed with food-safe silicone and secured by nut on the inside.
Many thanks @Wynne - took me a while to work up the courage to Google "running nipple" but I'm glad I did, which doesn't happen often.
The parallel nipple I got from pipe dream fittings, I believe.
Cheers for the eBay links @jjsh that's helpful. The prices may not be any better than Screwfix but I do have more faith in the accuracy of the descriptions, even if there's a bewildering array of options on the page at the end of that third link. Let me see if I can work out what I need before I hit you up with more n00b questions.