Advice on secondment whilst on a service tenancy?

I started a 6 month secondment for the same local authority on 31/08/09, it is the intention for me to return to my substantive post at the end of the 6 months. In my service tenancy it states that any changes to rent must be given at least 4 weeks notice. I usually pay 50% rent and full heating, hot water %26amp; service charges. My employer has now given me 2 weeks notice that I will now have to pay the full rent whilst I am on secondment, any advice is appreciated, does this make me a secure tenant? Can they do this? Etc. Thanks