Mobil 1 at £36.99 wih free p&p. It's about a tenner less than Halfords at the moment (although you do get a free Mobil 1 brolly with every purchase, if you'd prefer )

MOBIL 1 - 0W40 New Life Fully Synthetic Engine Oil - 5 Litre - 5L :: £36.99 :: Oil, Lubrication, Fluids & Liquid Additives :: MOBIL ONE - Oil, Fluids & Lubrication ::