Contract working hours HELP!

Greetings!

Could someone with more knowledge advise me on the following dilemma?

1) I joined my current Company in August 2008 and signed a contract to work as a Data Migration Analyst.

2) I was placed in the service desk team for the first three months with the promise of moving onto my proper role after the probation period.

3) The 3 months turned to 6 then 12 months and now 3.5 years later I'm still doing the service desk role (but getting paid the same as a data migration analyst.

4) Since 've been working in the service desk team, we have a rota whereby we have to work 2.30PM-10PM once every 6 weeks. This is not something I want to do but have been doing it for 2 years anyway.

6) We also had on-call night shifts (once every 4 weeks) where we got paid £100 a night which has now been taken away from us and transferred to the support centre of the parent company!

7) I have informed my manager that I no longer wish to do the late shifts (2.30-10) as normal support hours end at 6 and the only reason I did the late shifts was to get the on-call money. Now that has been taken away I is to work my core hours as stated in my contract.

8) HR have told my manager that I have no choice and I have to work 2.30-10PM but I've objected to this and wish to take this matter more seriously.

is there anything I can do or is the fact that I've done this role for so long means that I have in effect agreed to the service desk contract?!

Please advise if possible.

Thanks in advance.