Ride out, engine note changed and excessive popping on overrun, stopped and engine was well hot, s/steel exhausts changed to straw colour
Left to cool, restarted but very rough running and unable to pull off. Changed the plugs and boy were they hot, still no improvement, so for the first time admitted defeat and arranged recovery to home. Recovery was via flat back truck with tipping platform, certainly an experience, but the driver was excellent.
Commenced strip down and suspect carb leak at manifold joint, introducing air resulting in weak mixture, awaiting some new bits, and hoping no engine damage.