They want to change the mortgage over as I don't reside in the property anymore. An admin fee is going to be charged....... What do you think?

Basically a year ago I had to rent my property out as I had to move in with my mother as she had just lost her husband(my dad) of 42 years. My mother has mutiple sclerosis and is severly disabled. She requires 24 hour supervision. I informed the lender for my mortgage (Abbey National) there and then. I changed my insurance over to a specialised landlords insurance. I however have a redirection which is coming up soon ,so I thought I would tell the abbey. I have and now I've had another letter stating I never told them etc. I have spoken to customer service and have done a questionairre over the phone regarding why I have moved etc. I have been informed that there will be more than likely be an admin fee. A figure of say £100 to over a £1,000 has been banded about but as stated by the adviser each case is done on it's own merits..... Can someone out there give me an idea of what I will expect over the next few days ? I have moved for a purpose not to make money! My mothers health is far more important than having to deal with this possible burden... I would appreciate your help. Thanks.