I had a similar problem and after repeated visits to a "specialist" who kept replacing the gasket between the fuel tank and carburettor I decieded to have a look myself. Indeed the gasket had blown again and I found that the face of the carburettor was warped. I flattened it off with a file, washed it out with clean petrol to clean out the swarf, fitted a new gasket and it's been ok for a summer and a half. hope this helps.
Paul
September 2008