bluesmile, well done for getting some legal advice. I used to do IUC's for a government dept (not a Local Authority) and I will tell you this. they should not ask you "leading" questions but obviously any answer you give could alter the line of questioning. a copy of the tape will be available to you, and you will be given a note to explain how you can request a copy.
if you phone, they can not discuss the potential offence as you will not be under caution. when they call you in, you will be cautioned and the interview will be taped. you can take a friend if the legal advisor won't/can't attend, but they can't speak for you during the interview.
anyway, give me a shout if you need any more info.