Thank you for your suggestions.
If I put a non-breaking space between Le and Plessis, the Le goes over to the next line and spacing is worse!
It's interesting that Joel doesn't get the same result as I do when he typeset the example. The forum doesn't allow me to upload a file but I'd be happy to email anyone a version if they are interested.
There are capitalised words I want to hyphenate and turning that off or on doesn't affect the outcome.
If I stretch the text frame arbitrarily wider, at some point the whole of 'Plessis-Robinson' goes over, never 'Plessis-' on its own, which is the nub of the problem.