MrsD-S - I would definately agree with JonG, get yourself checked out before you apply - better to know sooner rather than later, even if it isnt what you want to hear. (sorry for the pun!) It would be awful to make it all the way through the selection process and then fail at medical.
Liamkirkham - You do not get asked anything about visits to ATC units at stage 2 - that is saved for the lovely couple of hours you spend with your interviewers at stage 3! It is more worthwhile to wait until after you have passed stage 2, firstly to make sure you pass your stage 2, and secondly so that everything is fresh in your mind. When you do visit a unit, I would definately recommend visiting at least 1 area centre and 1 aerodrome tower. It gives you a lot more to talk about and also a lot wider veiw on what you might like to do.
Best of luck with it all,
BW