The ST tank is the only one to go for. The R65/45 tanks look very similar but have a different fit, you'll need to take a hammer to the underside to get them to fit past the electrics tray and even then there will be a slight conflict with the rear of the tank and the side panels. The rear of the tank is also slightly longer forcing the nose of the seat back.
Not the best shot but you can see how the sidepanels just touch the 65 tank. The seat looks slightly baggy where the nose of it presses against the tank. The ST tank on the other hand is a perfect fit.
I put this on as a temporary while I got a leak fixed on my ST tank, unfortunately it's still on there.
Nice bike BTW The Basics are much sort after machines. They have a unique rear subframe that's similar to the early Mono one but made from stronger tubing and with tubes in slightly different places to allow the paralever mudguard and side panels to fit. this is the Basic subframe next to a mono one.