Shares in upscale department store group Selfridges soared on Thursday after it revealed it had received a takeover offer from a mystery bidder. The firm, best know for its famous Oxford Street store in the heart of London, saw its shares soar almost 20 per cent after it released a statement saying the offer was made at a "substantial premium" to yesterday's closing share price of 244.5p - valuing the company at £378 million.