My thanks go to Mike Butler for these photographs of a 3-wheeled Reliant Speedboat. Mike writes:
... attatched are a couple of scans of our local (Oxfordshire -UK) 'Reliant Speedboat'. I have no details but it's obviously on a Reliant chassis. Perhaps the owner will contact you with more details ... sorry about the quality of the scans, had to use the wife's compact camera!.