The owner did indicate (before he stopped speaking to us) that he did use the heating oil and I think, given the age and comments above, it probably worked ok on the French heating oil. I'll stick with diesel just in case though. Saving a few Euros at the risk of a big bill isn't worth it.
I've looked further in the invoice that was waved as evidence by the owner and it was dated 2014 ands vaguely states engine work. His comments re brakes (and possibly clutch) indicated this was done in the last year. I'm going to contact the servicing garage to see if they have history they are willing to share. But regardless of all that, it seems to be a low hour model in very good condition. And a new owner that has no idea how to use it yet, beyond driving slowly around the courtyard on the flat. Next step, hook up the lawn mover attachment and see what happens.
Thanks for all the comments. If I don't get on it with it, it'll be cleaned, listed and exchanged for a modern park mower

. It should be owned by someone that loves these models.