incidentally, browsing the M&P site, I noticed they are stocking 20W/50 oil now.
Motul (French company that supply Yamaha) branded. Mineral is something like £20 for 4 litres. They have a semi-synth flavour too.
Some other brands available too.
http://www.mandp.co.uk/products/list/Lubricants/4-Stroke-Oil