Can a company go back on a volentary redundancy agreement after its been agreed?

Hi there,
I work for a private company that is contracted to the Home Office.

I recently applied for volentary redundancy, after this i was suspended without prejeduce pending an investigation into an allergation.
Whilst on suspension I had a meeting with a director of the company who told me my request had been accepted and i was given the contract to get a solicitor to go over and also told my end date of employment was jan 4th 2012 along with the amount of money i would be getting. I have this in writing.
The day after this meeting I had an accident at home which resulted in me being signed off by my gp.

I did get the contract looked over by a solicitor and also returned it to the company last Tuesday. They have now hand delivered me a letter saying that they are unable to proceed with my application for volentary redundancy until they can investigate the allergation and determine whether disciplinary action be taken against me.

My question is can they do this?
Many thanks

Yes.

Redundancy - voluntary or otherwise - is not "contractual", and can be postposend, delayed or withdrawn. You are still under your employment contract and if an allegation has been made then it is reaosnable for the employer to wish to investigate it.