Thank you for the suggestions. I have been thinking of raising the offending section of the path by about 30mm with an appropriate coating.

In the meantime, I am still thinking about all options for front wheel braking, as I think this is still the most effective braking. This is a bit 'left field' but I was wondering about a manually operated brake that applied pressure to the ground between the front wheels. I think it would look something like this Manual brake There seems plenty of scope to mount it. Actual braking mechanism could be something like Brake Biggest problem I see is a mechanism to apply pressure to brake to force it firmly to ground.