MOT test class and catagory

Hi everyone I have a bit of a situation and need a little knowledge, 
I work at a class 7 mot station, around a year ago I was  presented with a prisoner transport vehicle weight 3.500 tonnes and has 5 passenger seats,
Thats three prisoners two guards and driver. I was told by a tester at another station is should be tested as class 4 as it was built for and its purpose is 
Prisoner transport so its not class 7 and tested as class 4.
It doesn't there last week I was presented with a larger prisoner transport this time ita 6.500 tonnes 8 passengers thats 6 prisoner's 
2 guards plus driver, now this one is obviously lorry based so the catergree should be N2.
With all the confusion the workshop manager spoke to DVSA who informed him that is class 4, didn't sound right for some reason,
Not wanting to test out of class I gave them a ring and was told by there technical support that if the vehicle  was made or converted for the sole purpose of 
Prisoner transport it is not a commercial vehicle so can be tested as class 4. That was almost word for word that the other tester said and looking at things I'm still not 100%. The presenter is a well known van and truck company so don't think they are trying on to save money.
Can anyone shed some light on this issue.

Many thanks