I have just had the new size Blockley 450/500 x 18 tyres fitted to my NB and I have found 26psi seems to suit the car best. Anything much above that and the ride becomes very harsh!
My PA has been on Blockleys for 2 years now and I have found that 28 to 30 PSI is fine. Must say that they have improved the handling and ride drastically.