Given that you only need to touch these nuts once a year at most is it worth buying a special spanner? I carry an adjustable spanner on the boat for these nuts and others of a similar size. From memory, the nuts are between 25 and 30mm if you want to size an adjustable.
I'm guessing it will be 24mm(M16 thread) - I looked at a Volvo a couple of days ago(not sure of the model, it was on a 28' boat) to do adjustments and that was the size(the adjustable was too bulky!) - definitely a job for a 24mm combination open end/ring spanner, a cheap one from the market will do the job.
Sorry but we have been through all the jokes and wisecracks about Whitworth, metric and left handed adjustables before until it wrecked some poor sod's thread. /forums/images/graemlins/grin.gif
Those old square jawed ones, on the right in the photo, are excellent for some purposes, where the angled ones won't fit. AFAIK they are not made any more, so never throw one away! Some day you will need it!