1) relate the above to your boss. If you get no change then
2) relate the above to your boss's boss. If you get no change then
3) Speak to the relevant HR Manager. If you get no change then
4) Raise a formal grievance quoting your contract of employment, relevant policies etc
5) Whilst waiting for the above to resolve itself (or perhas in advance of the above) test the market - what are your chances of getting a similarly paid position
6) Claim breach of contract/unlawful deduction of wages and submit application to ET
I am now going to stage 4 as of Monday.