BS112-12.37mm x 2.62mm (1/2" / 12mm) Imperial Size Black Nitrile Rubber"O Ring" / Washers x 500, to Suit John Guest Fittings Amongst Other Things : Amazon.co.uk: DIY & Tools
O Ring Kit, Inch & Metric Sizes, Nitrile-70A, Rubber Seals O Rings & 4 Pieces Orings Remover Tools in a Storage Case, for Faucet, Plumbing, Automotive (Red & Yellow Case) 786 Pieces: