Well what a difference pumping the tyres up makes! I have some new Schwalbe Marathon Plus tyres and had them at 60psi yesterday. In all that wind it felt like treacle and I struggled to get my average over 12mph for the 20 mile circuit.
Today I put the tyres up to the recommended max (70psi), did a similar route and averaged 13.4 mph (with slightly less wind, admittedly). The bike felt MUCH better, if a slightly rougher ride.