A former Post Office investigator has said she could not question Horizon system experts because she believed that “if you started to challenge too much, it didn’t go well”.

The Post Office Horizon IT Inquiry heard that investigators, such as Northern Ireland-based Suzanne Winter, were ranked from one to five based on their recovery of losses from subpostmasters.

Ms Winter was involved in the criminal probes of Belfast subpostmaster Alan McLaughlin and Maureen McKelvey, who was based in Omagh.

Mr McLaughlin was wrongfully convicted of false accounting offences in 2005 following an alleged £10,000 loss at his Brookfield Post Office branch in Tennent Street.

Ms McKelvey reported £30,000 shortfalls in the Horizon system at her branch in Clanabogan and despite eventually being found not guilty, the stress of her five-year wait for the acquittal caused her thyroid to rupture – resulting in major surgery.

Speaking about the performance ranking system she and other investigators were judged on while giving evidence on Friday, Ms Winter said: “We could see everybody else’s, but… I didn’t go looking to see what is everybody else doing.

“I was just concerned with what I had been targeted to.”

Ms Winter said she found some statements submitted by Fujitsu staff to assist her investigations “hard to follow”.

She said: “I did not have access to anyone in Fujitsu, or allowed to have access.

“I had to put my request through casework if I wanted anything from Fujitsu, and as far as I am aware, casework then dealt with that.

“About nine years in, our casework team then got a Fujitsu liaison person, and that is the person that we would deal with then if we wanted anything from Fujitsu.”

You just got the impression that if you started to challenge too much, it didn't go well

Counsel to the inquiry Emma Price then asked: “Since the statements related to your investigations, did you ever seek clarification in respect of the bits which you found hard to follow?”

Ms Winter replied: “No.”

Ms Price continued: “Why not?”

Ms Winter responded: “Because it seemed to be the technical side – they were being reported as the expert of the computer and you were more or less, in the Post Office where we were, if you challenged anything… you didn’t feel you could challenge anything.”

Ms Price then said: “Is there any particular individual or individuals who made that the case?”

The former Post Office investigator replied: “No, I wouldn’t say any particular individuals, but you just got the impression that if you started to challenge too much, it didn’t go well.”