Bit late for Ted I know, but might be useful for somebody else. I'm not fussed by having a ribbed front tyre so read this in that light.
I've used Avon Roadriders on a classic and one pre-war bike with good success covering 10s of thousands of miles in all weathers. I'm used to these tyres so they were an obvious fit on the 400/4 for me. 90/90 x 18 front 100/90 x 18 rear with no clearance issues with standard tinwear. They do give a 7% increase in rolling diameter for the front and 1% for the rear according to my sums. Change in diameter appears to have no effect upon the function of the main and side stands. When I checked the speedo with my GPS it was fine anyway, might be different for another bike. Absolutely fine when riding from walking pace to flat out on all the surfaces I've been on. Bend swinging is no problem and I've had no problems with stability under braking.
