Well the seller (very kindly) got back to me and told me that he had bought the adapter from these people
Dintra Transmissies in Putten, het adres voor aandrijftechnieken voor jachten en schepen
They do loads of nice coupling stuff too.
Just for the record - tthe English language catalogue is a bit out of date; but the most recent Dutch version has loads more stuff in it.
The adapter plate can be found on page 65 of the Dutch version of the catalogue for the price of 280 euros {and that's probably a price without 21% sales tax}
(So if "anyone" wants the one for sale at 79 euros + 13 euros to get it to me and then wants to pay for additional postage from me to you I'm happy to act as a go between so long as I know you - you also need to do this in the next 48 hours or so - otherwise someone else might snap it up)