I can't offer real advice, but you have a right to know why you are being seen and the scope of the interview. You could start with the care home staff, and if they don't know then call the number on the letter, or ring the number from which you were called. Ask who is coming, what is the purpose of the interview, and what information will you be expected to provide. They should give you enough notice to get this in order, so if the interview is on Monday, you can ask for it to be postponed for a week or so.
Obviously you can't conceal information, but it would be a good idea to understand the system, so maybe call Age UK or the CAB for advice. You will have a right to take someone with you too. Good luck.