SANDI EASTON, the first husband of singer Sheena Easton, died of a heart attack. Mr Easton, 49, who battled for years against depression and alcoholism, was found dead in his council house in Stenhousemuir earlier this week. It is thought he had lain undiscovered in his home for three days. Police prepared a report for the procurator-fiscal, who ordered a post-mortem examination. Mr William Gallagher, the fiscal at Falkirk, confirmed yesterday: ''The cause of death was coronary thrombosis.''