Just searched to Bromtontalk forum a bit; it is mentioned that the chainstay bridge is a problem along with the mudguards.

Well my chainstay bridge has about 13mm clearance on the Kojak an the closest chainstay about 5mm.

The mudguard has wide clearance all round except at the back - the screw that holds the flap would rub but that I believe is easily remedied. I am beginning to think Brompton has modified the design to accommodate wider tyres. So I am going to try a Scorcher.