Don't do a deal in Dover, you don't know anything about the buyer, and all this transfer part-funds malarkey is a disaster waiting to happen.
Assuming the buyer is genuine, then if he's prepared to come all the way from Germany for a £15k boat, then what's a few more miles to the final destination at the dry storage yard.
If it were me, progress the sale with plenty pictures and a comprehensive description so he's under no illusion as to what he's buying, look out your own paperwork, and draft a receipt to be signed by both parties.
The buyer comes from Germany, pays cash which is transferred into the bank before the release of the boat. Hell, you could even book him to a Travel Lodge before the return journey! Good luck with the sale and let us know the outcome.