Warren Buffett, a major American investor, has filled his trolley with £350m-worth of shares in Tesco's, the UK's largest supermarket chain. The news did not come as much of a suprise as the business mogul owner of investment vehicle Berkshire Hathaway had said towards the end of 2011 that if Tesco's share price fell he would buy up more shares. After the £5bn nose-dive in Tesco's shares last week, following the firm's worst festive period in 20 years, Buffet has stayed true to his word and stocked up, significantly increasing Berkshire Hathaway's stake from 3.6% to 5.1%. Buffet previously purchased 34m Tesco shares at 371p each in September of last year. In a post-Christmas statement the company admitted that not only had it been its worst fetive period in decades, but UK sales were falling and hundreds of millions would need to be spent on improving UK stores. The announcement sent Tesco shares up 1.75p to 322.75p.Top Director BuysCoastal Energy Co (DI) (CEO) Director name: Mr Olivier De MontalAmount purchased: 10,000 @ 1,024.70p Value: £102,470Sierra Rutile Ltd (SRX) Director name: Mr Michael BartonAmount purchased: 20,000 @ 55.00p Value: £11,000Top Director SellsCoastal Energy Co (DI) (CEO) Director name: Mr Lloyd B SmithAmount sold: 3,000 @ 1,025.34p Value: £30,760NR