Thats an interesting question dc, I'm self employed but was a JIB spark years ago. It may say something about that on your Contract of Employment, if you have one. The JIB rate is also the union rate I would guess, but no one is in the union anymore. You could phone the JIB or the ECA ( who own JIB) . Work is very scarce at the moment so I would be careful how you approach this, hopefully they are not just taking the urine. They may be hoping that some of you will leave !
I always thought that they had to pay the basic rate, could be wrong, but you may have to accept it to remain in employment.
Sorry I can't give you a definate answer,
Deke