In reply to Mooncat:
Although its a bit complicated to explain there does not have to be any consultation at all in that if your employer changes the contract in anyway and you continue working (there is no such thing as working under protest), the relevant case law is quite clear. If you continue to work then you have been seen to accepted the changes.
However:-
In general, to achieve a dismissal free detrimental change to employee's contracts (no one objects to positive changes normally!!), then case law expects you to have consulted and tried to reach agreement. If no agreement is forthcoming then you'd normally dismiss for 'Some Other Substantial Reason - SOSR and issue another amended contract. This would normally be the safe and accepted - and of course fair way - of changing the contract.
But from an employee's point of view you still have very little way of doing anything about a negative change unless you are willing to resign and claim you were constructively dismissed (breach of contract) in that the breach was a major one, enough to make you walk out immediately. (and if you stay you will be seen to have accepted the breach) And I doubt in this case that being asked to give extra notice, when, as I said before you cannot be held to this anyway, along with the fact that it would only come into force when you actually left makes the whole affair a non starter.
You are correct of course, all employees have contracts' whether they are written down or not, signed or whatever. The law requires that you are given within six weeks a written statement containing the main terms of the contract. Many employees don;t ever receive one, but non the less they do have a contract. The minute you accept an offer of work a contract of employment is made.
One other very good reason why you cannot be forced to work your contractual notice is that it would mean you are being forced to work when you don't want to. And slavery or forced labour was officially abolished a couple of hundred years ago.