Problem resolved. Thanks Roger for the attempt, and the problem was due to an air leak, but not in the carburettor diaphragm. The carburettor glides over a sleeve protruding from the cylinder barrel and the annular gap is sealed by a rubber "O" ring held in place by a plastic ring just inside the carburettor orifice. The ring had become displaced, allowing the "O" ring to move out of position and thus admitting air giving rise to the irregular engine note. When the seal was put back in place, the engine ran smoothly once more. Tip: reconnect the hair spring above the carburettor before reattaching the carburettor to the cylinder barrel - it makes life much easier than trying to connect it afterwards.
Whyard
September 2012
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