In London, the FTSE 100 index dived back below 4600, down 50.9 at 4564.5. Turnover was swollen to 1.3bn shares by midday following the buyback in Marks & Spencer shares. The tender offer for 635.3m shares, or almost 28% of issued capital, was pitched at 362p, just above the middle of the quoted range. US investment bank JP Morgan has downgraded M&S, 1 1/2p higher at 358 1/2p, from overweight to neutral with a fair value price of 400p.