Tim, would you consider a tank from a Japanese bike? If so, you could visit D & G Motorcycle Breakers, Clothall Bury Farm, Clothall, near Baldock SG7 6RJ. They have a shed full of tanks from 1970s, 1980s bikes. If you can take the frame down, they'll help you find a tank that'll fit the Greeves frame. When I was down their place the prices were reasonable.
The Greeves has a single tube spine as do a lot of bikes from that era so your chances a good.