thanks to everyone for the useful info. I will pass it on to my nephew.
The brokerage who sold him the boat advertised it as a Watercraft Boat built in 1976.
I did manage to contact David Harris who worked at Watercraft in the 70,s and he had no recollection of it or of them building a boat with a 5LW.
I recently accompanied him from the Anchor Inn at Cross Green down to the Bratch. The boat swims nicely and the Gardner runs cleanly once warmed up. Typical of all 5 cylinder engines it runs very smoothly (no need for a magnet to hold your tin mug on the cabin roof) The gear box is a little noisy and neutral not easy to find so I will be helping him strip it down shortly.