Update on my issue. I had done all my checks underneath the car with it in Park. When I moved the selector to the other end of its travel the problem became clear. The kickdown arm was fouling the gear selector. With the selector in Drive - or worse, second, and the throttle pushed hard down, the kickdown was just able to push the gear into neutral.
As far as I can tell no harm done. The gear selector arm has now been bent down and now all linkages seem to play happily together.
When I started to bend the gear operating arm I did it cold. This was a mistake as it started to crack. A brief application of the welding torch followed and now all is well.