Changes in terms of employment and right to negotiation

Which Act(s) (if any) prevent an employer unilaterally changing terms and conditions, particularly calculation of pay and bonuses?

From Citizens Information:

... changes must be agreed between your employer and yourself. Neither party can unilaterally decide to change the contract.

Click to expand...

I've looked at the employment Acts referenced on that page and can't find this specific provision. Is it a general principle of contracts rather than employment law?

Thanks Froggie -- but that deals with methods of payment and unauthorised deductions. It doesn't (seem to) deal with the the terms and conditions surrounding the calculation of wages.

Might be worth contacting these guys.