The Serious Fraud Office has sentenced three former senior executives of Welsh Slate to jail after revelations that they had deliberately overstated the company's accounts over a number of years. Auditors of the company were duped by the former executives, who showed them stockpiles of crates of roofing slates - many of them were actually empty. They also produced customer letters that made out debtors' payments were in the pipeline, but the delivery notes and transportation invoices for non-existent consignments were fake, the SFO said. In total, around 44% (over £10m) of the company's reported debtors weren't real. Managing director Christopher Law, operations director Geraint Roberts, and Paul Harvey head of sales have been sentenced to two and a half years, 16 months and 10 months' jail respectively. Welsh Slate is a subsidiary of Alfred McAlpine, a self contained operation that was seen as 'low risk' by its parent company. It was in February 2007 that Alfred McAlpine blew the whistle to the stock market that it had discovered systematic deception at Welsh Slate. An internal investigation ensued which led to a joint investigation by North Wales police and the SFO. 'These three defendants collaborated with each other over a period of years to falsify the invoicing, management accounts and audited accounts of Welsh Slate,' the SFO said.