When you say "De winterised " What was done , it could be a number of things but i have noticed that the warning light takes a while to go off on mine if its been stood a few months might just be a sticking oil sensor & if the engine wouldn't run fast enough to put the light off .
Also If fogging oil was squirted into the carb to winterise & the plugs haven't been cleaned it could have just clogged one up until it burned the oil off or gummed up one of the jets in the carb until the petrol dissolved it .
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