TOM Nelson works with the police. But not for them. And his keycard proves it.

Scotland’s director of Forensic Services is standing at a door officers cannot pass. It leads to his labs at the national crime campus at Gartcosh, a building otherwise brimming with police. “This is for our staff only,” he explains with a gentle nod at locked entrance. “We are really keen to ensure our impartiality and independence.”

Mr Nelson has a point to make. England, after a decade of cost-cutting and partial privatisation, is facing a crisis of confidence in scientific evidence. Some 10,000 motoring offences - and 600 rape and sexual assault cases - are in doubt after a series of scandals and controversies involving both digital and biological forensics.

In contrast, Scotland has kept a national service, under the Scottish Police Authority or SPA , rather than Police Scotland.

“We were very keen to have a differentiation - a sterile corridor - between the police investigation and the scientific investigation,” Mr Nelson explains as ushers me in to his labs. “Because we are not just here to prosecute the guilty. We are also there to exonerate the innocent.”

Could Scotland’s national forensics service come to England’s rescue? Fully accredited (unlike all police labs in England), Mr Nelson’s team is looking for a potential market down south. They have already carried out case reviews. Now they are looking to see whether they could sell their expertise on the free market. But not just yet.

The forensics service has a nine-year plan to take them, with the rest of Scottish policing, to 2026. Its unique offer? A “crime scene to court” single service already attracting attention as a model in Ireland North and South.

Behind his door, Mr Nelson’s ‘sterile corridor’, one of many housing some 200 scientists and staff in the forensics wing of Gartcosh, is spotless.

The air is thick with flower -flower-scented detergent. Huge smearless windows separate labs from the corridor. Technicians in white coats and hair nets work at white tables, There is sticky paper on the floor to stop them tramping dirt from one room to another. Mr Nelson stops at one window where three big glass boxes stand. They’re robots, he explains, for doing DNA tests, 96 at a time.

One of the samples, soon, will be mine. Just to get in this lab and talk to Mr Nelson, I and photographer Colin Mearns had to brush our gums with little combs to catch our saliva. (My first sample was espresso-stained. We took two each. Caffeine, it turns out, can contaminate DNA.)

DNA technology keeps changing. Scotland now uses a DNA 24 standard - meaning they can identify 24 markers. England and Wales have DNA 17, slightly less detailed. Both standards, however, represent huge strides compared with just a few years ago.

Mr Nelson said: “We can go back now and re-run samples which are four or five years old in the old system where did not get a profile that was reportable with and still get statistics which are more than one in a billion.”

The rise of reports of sex crime are generating more and more work for biology labs. A complainer could come forward days after an incident. In the past that might mean vital clues are lost. Now, with DNA 24, even washed clothing, in the right circumstances, can give up evidence.

New technology is also allowing Forensics to provide new intelligence for the fight with organised crime.

Late last year a gang handling £100m of drugs a year was destroyed in what the police called Operation Escalade. Nine of its members were locked up for a total of 87 years

A key piece of evidence: DNA captured by Gartosh scientists on the grip, barrel and chamber of a Beretta that two years earlier had been used to pistol whip a man called Robert Allan, who owed the group money. The gun had been washed.

Mr Nelson’s team processed 1000 DNA samples and another 1000 fingerprints for Operation Escalade alone.

Mr Nelson - who cut his teeth as a chemist investigating explosives in his native Northern Ireland during the Troubles - is careful in his criticism of what is happening elsewhere.

“Our model is very different here. I believe our model works. We have quality and accreditation at the heart of what we do. I am concerned that that same heart isn’t in all aspects of forensic provision in England and Wales. They have probably commoditised the market and put a price on everything.”

Mr Nelson stressed it’s hard to measure the financial value of proving a suspect innocent.

He has ambition to get work for Scotland down outside the country. “How can we take our model out and help other places? Can we do some income generation? But that is at the end of our nine-year plan.”