By Laura McGuire Superdry blamed unseasonable weather for a 13 per cent slump in retail sales year-on-year, as the struggling fashion retailer continues to trade below management expectations. Wholesale earnings were down 41.1 per cent year-on year, which was due to the decision to exit its US wholesale operation. It was also driven by “timing differences and the underperformance of the channel”,a full year trading update published this morning said. Julian Dunkerton, founder and chief executive of Superdry said: “The un-seasonal weather through the early autumn led to a delayed uptake of our …