Employers NI

[hackers]

Hi

I have been working through a recruitment agency for a major telecoms company on a rolling contract for nearly twelve months.
When I first started I was told I would have to to work through a payroll company to which I wasn't entirley happy about at the time but I grinned & beared it.
Some of my colleuges however have been on PAYE direct for the agency at the same time, with the same hourly rate as me.
At the turn of this quarterly contract I was told that I am to get a payrise of approx 12%, when I asked what this was for they replied it was towards my employers NI.
My question is shouldn't I have been recieving this from the very start of my empoly & is so are they obliged to back date it.

Thanks in advance.