Detective Sergeant Geoff Ferris, and co-complainant Conor McStravick, have their case listed for hearing in Belfast on Friday.
Mr Ferris was a prominent figure during Stewart’s trial at Coleraine Crown Court in 2011.
The mother of two denied being part of a joint enterprise with her then lover Colin Howell to poison her policeman husband Trevor Buchanan, 32, and his wife Lesley Howell, 31, in May 1991 – crimes police concluded were suicide until Howell confessed 18 years later. However, she was unanimously convicted of both killings by the jury and is currently serving a minimum 18-year jail term.
Dentist Howell, from Glebe Road, Castlerock, is serving a 21-year jail sentence after pleading guilty to the two murders at Belfast Crown Court in November 2010.
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During one of his many interviews with Stewart in Coleraine station, Mr Ferris, who had described the crimes as “nearly the perfect murder,” put it to the suspect that Howell was not acting alone.
The detective said: “Colin Howell could not have done this on his own and you could not have done it on your own Hazel, this had to be a joint enterprise between the two of you, the two of you had to work together to make this plan come to fruition, do you accept that?”
Stewart replied: “Yes”
Mr Ferris added: “Sorry, just for the benefit of the tape.”
Stewart repeated her answer, twice: “Yes, yes.”
A spokeman for the Industrial Tribunal office said the case related to “public interest disclosure”.
A PSNI spokesman said: “It would be inappropriate to comment at this time.”
Geoff Ferris is also well known in football circles – mostly for his time spent at Glenavon where he scored more than 200 goals.