By Amber Murray Widespread retail discounting boosted online sales growth in September, but has continued to squeeze their already-tight profit margins, according to fresh data. Total retail sales grew by 4.7 per cent compared to September last year and online sales were up by 11.6 per cent, accountancy and business advisory firm BDO’s high street sales tracker found. However, in-store sales grew by only 1.8 per cent. “These sales figures may appear positive at first glance, but they show clear issues for the sector as we approach the most critical time of year,” Sophie Michael, head of retail…