Firstly, it's worth pointing out that a verbal contract is theoretically just as binding as a written one. However, in the absence of independent witnesses to a verbal contract, it's really what was put in writing that matters.
The law states that you should have been given a �written statement of employment particulars� within two months of starting work (or within two months of starting a new contract):
http://www.direct.gov.uk/en/Employment/Employe es/EmploymentContractsAndConditions/DG_1002790 5
The employer can't normally change the terms of that contract, without your agreement, unless there are collective bargaining arrangements with a trade union. (If so, your contract might be varied even though you're not a union member):
http://www.direct.gov.uk/en/Employment/Employe es/EmploymentContractsAndConditions/DG_1002807 9
There's prima facie evidence that the employer has acted unlawfully but that's based upon what appears to be a verbal agreement, rather than a written contract. (As previously stated, a verbal agreement theoretically carries as much weight as a written one but, in practice, it's almost impossible to prove exactly what was said).
You need to seek expert independent advice. Fortunately, that's available from ACAS by phoning their helpline on 08457 47 47 47 from 0800 to 1600, Monday to Friday. (Expect to spend ages getting through but it should be worth the wait).
Chris