MAHABA,
I'm always amazed when the 'mobile grade' issue gets thrown into the equation. When you joined and signed the peice of paper stating that the post was a mobile grade and you could be moved anywhere within the UK etc... why on earth did you not express your concerns then and explain to NATS that this was not legally enforceable!? Why did you sign? And what did you understand when you signed on the dotted line?
Would you still complain I wonder if you had failed a validation and NATS had the nerve to turn around and offer you employment elsewhere within the company - and dare I say - you had to move! Would you accept it? Ofcourse not, because NATS can't do that, it's not legally enforceable!