You have to be a maintainer of the project in order to be allowed to restart a Travis build

Ok - that would explain it.

Try closing and re-opening the pull request

Heh ... obviously you have a deep understanding of the basic principles of IT ;-)))
Thankfully, the PR in question has since been merged (and closed) ... and I aint messin wif nuffin. (One doesn't poke at Islam, and one doesn't poke at Github.)

Seriously - thanks for your response, choroba.
And thanks also to marto, for making me aware of the lower level possibilities.

Cheers,
Rob