Spy evidence so secret it's banned from court

Some of the evidence about Gareth Williams' work is so secret that the coroner has agreed to exclude it from the inquest under a Public Interest immunity certificate.

We don't know what the secret material is (otherwise it wouldn't be secret, obviously) but the certificate does shed some light on the reasons why Foreign Secretary William Hague believes it necessary to withhold it from everyone other than coroner Dr Fiona Wilcox.

Mr Williams' family opposed the application as best they could but it's not easy when nobody will tell you exactly what it is you're opposing in the first place.

We also know the general type of information it is from the ten page document Mr Hague has personally signed and which references previous court cases involving the intelligence services including the claim for compensation by Binyam Mohammed for being rendered to the CIA (a case the Government believes justifies secret courts).

The document also explains why the MI6 'corporate witness' and Williams' colleagues should have anonymity and give evidence from behind screens (which the Williams family do not oppose), and has some other interesting details too.

Mr Hague, who has personally read the material he has asked to be withheld, argues that some of the information relevant in this case carries "a high level of classification...revealing this material in open court would cause a real risk of serious harm to the public interest."

He gives six broad reasons why this is the case:

• risk of revealing secret techniques and technology;

• risk of revealing identities of people who assist intelligence services / provide information;

• risk of revealing identities of intelligence services personnel;

• risk of revealing details of intelligence service operations;

• risk of jeopardising relations with intelligence liaison partners abroad (basically, the Americans);

• risk of making it easier for terrorists / organised criminals to understand intelligence services;

There is a schedule of types of information each corporate witness will cover in their evidence by "articulating the combined factual strands", most of which can apparently be heard in open court but some will be kept secret and so should be covered by the PII.

For the GCHQ witness, who is Stephen Gale, former head of HR (apparently not anonymous as he has given evidence publicly in a previous case), it will cover the following points:

• functions of GCHQ and its relationship with MI6;

• Mr Williams' recruitment, career history prior to secondment to MI6;

• how Mr Williams was seconded to MI6 and what happened when he want to return early to GCHQ;

• Mr Williams' accommodation arrangements in London while on secondment;

• Vetting processes and how it was applied to Mr Williams;

• What GCHQ has learned about Mr Williams' 'personal life' since death and 'implications in context of security vetting' ;

• reasons why GCHQ don't believe Mr Williams' death was related to his work at GCHQ;

• actions taken by GCHQ ' in days after learning of Gareth Williams' disappearance';

• actions taken by GCHQ after Mr Williams' death regarding vetting, absence reporting and procedures for other postings.

The MI6 officer giving 'corporate evidence' anonymously will be screened from public but not lawyers or family.

She served from March 1983 to March 2011 and was a director first of HR, Finance & Training and then Operational & Personnel Security including vetting and parliamentary oversight / public affairs.

Before that she was operational officer with foreign and UK postings.

Her evidence public and secret will cover the following areas:

• functions of MI6;

• circumstances and nature of Gareth Williams work at MI6;

• matters relating to security and vetting;

• matters 'that have emerged since Gareth Williams' death';

• whether MI6 see any connection between Mr Williams' death and his work at MI6;

• events following Gareth Williams disappearance';

• 'lessons learned by MI6 in relation to the internal communication of MI6 absence reporting'.

This suggests both services are planning to hold their hands up to not chasing Mr Williams up quickly enough after colleagues reported him missing and admitting their vetting procedures missed his apparently developing interest in women's clothes and confined spaces.

But of course this actually supports their principle claim that Williams death had nothing to do with his work and was a tragic accident.

It also suggests, however, he really was very important to the intelligence services given the number and nature of the reasons cited for keeping his work secret.

At Mr Williams' funeral, MI6 chief Sir John Sawers personally paid tribute, saying the "highly talented... generous and modest" mathematician had done "really valuable work in the cause of national security."

If nothing else the lengths the Government has gone to keep Mr Williams' work secret suggest they were than just fine words.