Apart from calling the Alzheimer's society help line, you might also look at the mind site mind.org.uk. You will have to be clear when you talk to anyone that your relative has not been detained under the 1983 Mental Health Act, but the inssue is the social worker's interpretation of the mental capacity act. This act is so new that it will be difficult to find any real informtaion about it. However, the guardianship website may have something (
http://www.publicguardian.gov.uk/) on their brand new web site.
To be honest I've only had a quick look through but nowhere can I find the term "Decision Maker". There are court appointed Deputies, could that be it? I do not believe it is within the power of the local authority to make this decision, only the court appointed deputy can do that, so I would start out by asking for a copy of the court order.
Edited to add:
Having had another look, I see that section 5 gives the option for informal decsion making which seems to be what has happened here. However it also says that a formal application is appropriate if there are disagreements that caonnot be resolved. It also says you can contact the court of protection if you need to ask advice, and I would suggest you do that first. Actually what it says is
There are certain situations however where informal decision-making is not appropriate, and an application to the Court of Protection is necessary. This may include for:
• particularly difficult decisions;
• disagreements that cannot be resolved; or
• situations where ongoing personal welfare decisions must be made about someone who lacks capacity.
I have to say that sounds like your situation.