A very large can of worms can be opened on this one! The situation is covered by the Employment Rights Act 1996. (ERA96) The act is vague and confusing but has some key elements. The main one being that a person cannot be made redundant, it's his or her position that is redundant. Therefore if a person is 'dismissed for reasons of redundancy' that person would have a reasonable claim for unfair dismissal if some is re-employed in their place within a reasonable period of time - usually considered to be 6 months. There is no mention of if that person is re-employed in their original job.
There have been test cases in the past - a lot of public sector workers were laid off in the nineties to be re-employed by sub-contractors on lower pay. I don't remeber how they fared.
However, to me it sounds like you have a case, but an employment tribunal wouldn't touch it as you haven't actually lost you job therefore it would have to be a civil case. You can bet your employer has consulted his lawyers on this one and feels he acting from a position of strength.
Am excellent bookregarding all aspects of the law is 'Law without a lawyer' by the barrister Fenton Bresler. (ISBN 0-7126-8091-8). It has an excellent section on redundancy and employment law.
My advice, consult BALPA.