Not much attention has fallen on the role which the Justice Minister has in authorising electronic surveillance in PSNI investigations.

It could happen if, for instance, a phone tap or bug is requested in the police investigation of First Minister Peter Robinson's wife Iris and her financial dealings, or as part of the money-laundering investigation into Brian Arthurs, who has served as Michelle Gildernew's election agent.

The expectation of some politicians is clear and will test Ford's independence of mind to the core.

When Sean Gerard Hughes, a senior south Armagh republican who has been named by Peter Robinson in the House of Commons as a former member of the IRA, had his assets frozen as part of a money-laundering investigation, senior Sinn Fein politicians sprang to his defence.

"I know him very well. He's a good friend of mine and has been for very many years and I'm very proud of that," said Conor Murphy, the Sinn Fein minister in whose constituency Hughes lives. Mr Murphy blamed the "faceless opponents of Irish republicanism" for pushing the case.

Michelle Gildernew, the Agriculture Minister, was even more pointed, describing Hughes's arrest as "politically motivated" and adding ,"It strengthens the need to have devolution of policing and justice as quickly as possible. We need decisions, we need accountability, and we need decisions to be taken by a local minister."

As it happens, she was badly briefed. The action against Mr Hughes was taken by the Serious and Organised Crime Agency (SOCA) which, like MI5, is not accountable to our local justice minister. The rough rule of thumb that if something affects the security, or welfare of the UK as a whole, it remains out of reach of Stormont ministers, the Policing Board, the Police Ombudsman and other local scrutiny.

Dissident republicans and international terrorism remain the preserve of MI5 in its lavish new headquarters in Holywood, while SOCA handles alleged criminal conspiracies whose operations reach beyond Northern Ireland.

However, the implication of the minister's comments was clear: she believed that devolution of policing and justice should provide an avenue to bring pressure and accountability in such cases. It is not unusual for members of Stormont parties to be caught up in criminal investigations.

The political difficulties which could be faced by Ford are illustrated by the experiences of direct rule minister Mo Mowlam.

As Secretary of State, she authorised most of the phone taps in Northern Ireland including one, she later admitted, on the home phone of Martin McGuinness, who is now deputy First Minister.

When my wife Kathy Johnston and I co-authored a biography of McGuinness, we obtained transcripts of some of the calls. They recorded Ms Mowlam herself chatting happily to Mr McGuinness, apparently oblivious to the fact that MI5 and Special Branch were listening in.

She called the Sinn Fein leader "babe", as she did everybody, but more seriously she confided her differences with Tony Blair and said she was "fighting like f***!" to stay on as Secretary of State for Northern Ireland. She also advised him on the Irish government's mood. "If you are speaking to Bertie [Ahern, the Taoiseach] you should be OK," she told Mr McGuiness ahead of one meeting.

It is easy to forget about phone taps and that is why they are seldom discussed. Individual taps are so secret and so sensitive that it is a crime to disclose their existence, so we can expect little in the way of scrutiny. After we published the transcripts, our house was raided and we were arrested, but no evidence was found.

In Westminster there is a long-standing policy, the Wilson Doctrine, which specifies that an MP's phone cannot be tapped unless the House of Commons is informed. This seems to have been honoured in the breach in Mr McGuinness's case.

Will any sort of disclosure, or consultation, apply in Stormont if David Ford is asked by the chief constable to authorise the tapping of a phone used by an MLA or a party worker? What sort of strains would it put on Ford's relations with fellow members of the Executive if he knew about intelligence-gathering operations against them or their circle of supporters? Would he be able to simply put it to the back of his mind like Mo Mowlam?

When I talked about this issue to a senior retired police officer, he raised another scenario. "If Matt Baggott knocks on David Ford's door with a file saying that Patrick X has to be returned to prison and Mr X is a Sinn Fein politician or election worker, will he or will he not be returned to custody?"

This point came up in the case of Sean Kelly, the IRA man injured in the 1993 Shankill bombing when his device exploded prematurely. He got a life sentence but, under the terms of the Good Friday Agreement, was released on licence in 2000.

In 2005 Kelly was returned to jail for allegedly breaching the terms of his licence by rioting against the security forces. Peter Hain, the Secretary of State, revoked his licence, pronouncing himself satisfied that Kelly was "re-involved in terrorism".

There was uproar from Sinn Fein and Mr Hain released Kelly again on the eve of an IRA announcement that it had ended its campaign. The whole issue had become politicised.

After April 12, these decisions will be made by Ford. He will need integrity and a cool nerve to deal with cases which touch the interests of his political partners.

Last month he defused an awkward situation when he met the concerns of Martin McGuinness and Bloody Sunday relatives by apologising for his earlier description of the Saville enquiry as "pointless". It may not be the right thing to accede to pressure on the next issue where political interests are involved, but then, that is when his real test will come.