katie, re the first reply.. you will not be offered any free legal advice. your appointment letter should give you an idea why they want to see you (ie; regarding work, living arrangements, capital etc). if it doesn't give you any idea then ring them and ask.
also, they will not call you in for an interview under caution unless they feel they have evidence of an offence. evidence is not "we think you might be working because your neighbour said so". evidence is pay details from a company, bank statements showing you have capital, various documents showing that you may have a partner living with you.
if you are sure that you have reported any changes in your circumstances that may affect your benefit, you will be fine. they can't assume an income from an unpaid job. they shouldn't intimidate you. they will show you the evidence they have, and you will have an opportunity to explain anything that comes to light.