Help i have a job interview, but its not what i have applied for?

It may be wrong but just go. An interview is an interview on this current market. Also there is a chance that it the way they term it. You will have the chance to bring the question up and correct it at the interview. By then you may have made them interested enough in you that it does not matter.

Obviously you did apply for the costumer service job otherwise you wouldn't have been called. Is there a particular reason for applying to be general assistant? If not maybe customer service will interesting. If you really don't want the job just call them and tell them no thank you. You don't have to take the job.

go for it, these things happen for a reason, plus customer service looks so much better when applying to other jobs, also being a general assistant in sainsburys would mean stock rotation and kepeping aisles tidy, whereas customer service would be tills and sitting at refund desk etc. when you apply for office jobs in future, customer service stands out so much more then general assistant. don't be nervous in the interview, be yourself.