A man convicted of being one of Scotland's biggest cocaine smugglers is to appeal his case following the discovery of new evidence.

William Gavin Grant was arrested in 2002 and charged with trying to import one of Scotland's largest consignments of cocaine, worth an estimated £50m. He and three other men were caught by Customs and police officers in a complex sting operation.

The drugs, which the gang were transporting direct from South America, had already been seized at an English port and replaced with sand. When some of the group went to pick up what they thought was cocaine, they were arrested.

Grant, 42, from Bothwell in Lanarkshire, was accused of being a ringleader alongside James Mair, and they were jailed for 18 years each at the High Court in Edinburgh.

However, new evidence has come to light which suggests Grant had nothing to do with the smuggling, and Mair has written to him from Barlinnie Prison in Glasgow, apologising for the fact he was accidentally caught up in the operation.

The main evidence against Grant came from the testimony of Hannah Smith, Mair's secretary, and the analysis of a fax machine which officers claimed Grant used to organise the shipment.

The Herald has seen the transcript of a taped police interview with Ms Smith, which has only just been handed to the defence and highlights major discrepancies between her different statements.

In addition, lawyers and investigators have uncovered flaws in the expert claims that the fax organising the shipment came from Grant's machine and suggestions that evidence was improperly handled.

Evidence seals were broken on the evidence with no mention why, yet traditionally if officers need to remove something or open the evidence bag, they have to explain reasons and log the action.

Grant's appeal will be led on a number of grounds, including non-disclosure, insufficiency of evidence, and discrepancies in the statements of the key witness.

The case is likely to reignite the debate about non-disclosure. The Crown is still refusing to hand over the surveillance logs involved or allow access to the original fax machine.

Peter Ritchie, a retired detective superintendent from Lothian and Borders police who spent five years at Europol and worked as the head of the organised crime squad in NCIS (National Criminal Intelligence Service), has been working on the case for the past two years.

Grant, speaking to The Herald from Glenochil Prison, said: "The appeal was granted in January 2004 and I have been waiting since then to get a date set and to get the Crown Office to hand over the remaining documents that we need. The transcript of the police interview with Hannah Smith was only made available at the end of August this year and they are still not allowing us access to the original evidence to allow our forensic experts to look at it.

"It is just so incredibly frustrating. The strain it's putting on me and my family is incredible. My mum is really struggling to cover my mortgage payments and keep things going.

"I just want a date set so I can get the appeal heard. All I was doing was looking after someone's business while they were on holiday. This could happen to anyone - that's one of the scariest things about it. Mair wrote to me in Barlinnie saying don't worry about this, it's got nothing to do with you. But I'm still here."