Thanks for your comments folks.
It seems daft in this day and age that there's no central register of stolen boats and outboards. No wonder they're nicked so often!
Having spoken to the guy, it looks like I'm going to get a decent paper trail to look at tomorrow. It isn't on the SSR, but he has original bill of sale for the boat and the outboard, as well as service receipts and his current insurance cert. All being well, I should have my first RIB very very soon
The reason I was suspicious in the first place is that he kindly agreed to deliver the boat to my place yesterday, but forgot all of the paperwork on the day!
Cue Big Ben sized alarm bells! It does seem like a genuine oversight though - tomorrow will reveal all.