I've sorted it now
I was putting my acid above the radiator in my heated cabinet and then hanging the test piece above it and nothing was happening.
So yesterday I put a shelf in the heated cabinet just above the radiator so that I could stand a rusting box on that.
I used the plastic box I'd rusted my Original 5g in during the summer months so I knew the box worked.
I've just had a look in the 5g box and my test pieces are rusted to buggery so that shows me the way forward with the acid rusting --- You have to keep the whole setup warm from the outside.
So if you have your rusting box in your garden shed you'd have to keep the whole shed interior warm or build a heated box in your shed to put your rusting box in.
I can't see any need to play with other chemicals now to form the rust as the acid works fine if you get the right environment outside your rusting box --- besides which I'm getting a bit fed up of weeing on bits of metal.
All the best Mick