There have been any number of other threads about this.
I like Rory Stewart. I find his flights of fancy quite entertaining. However, he has a long history of saying outrageous things and then going very, very quiet when asked for his sources. This is one of them.
What I will say is that I have had professional dealings in the past with Steve Gillan, General Secretary of the Prison Officers Association and respect him as someone who is wholly steadfast in supporting and protecting his members. If Rory's claims were true, I would expect Steve Gillan to be shouting from the rooftops about it (and he wouldn't care whose toes he tried on as long as he was protecting prison officers). He hasn't.
If anyone can find a later article shedding more light on Rory's claims, I'd be glad to know of it because as a past member of the POA, I would be demanding to know what they intended to do about it.