There's a plastic cap you can get, intended for resealing opened beer bottles (why? why not just drink it?).
Only place I'd seen them was online at 89p for 10, but with prohibitive p&p.
But, I recently found out our local hardware store in our local market stocks a few homebrew items, so dropped in. Hadn't seen them there before so thought I might have to get them to go through their wholesaler's catalogue, but on checking the shelves, lo and behold - 4.95 per hundred. And no p&p.
They probably still sell nails by weight, too.