Thanks all. The tyres are almost new. I might look into changing them anyway, as this undermines my confidence! Manufacturer's website... well they no longer make this particular model. About asymmetrical tread patterns in general, they say this http://www.avon-tyres.co.uk/car/tread-patterns : So my conclusion is the tyres are fitted correctly, and either a) the precise pattern of all the diagonal grooves is irrelevant, and they have no role in water channelling, or b) the pattern does matter, in which case my RH tyres will behave better than my LH in the wet (when driving forwards!), and this tyre design is really dumb. Maybe that's why people find it's not got much grip, and why they no longer make them!? This design could only work if they made specific LH and RH tyres, with the diagonal stuff oriented the opposite way.