Will I need to pay any administrative fees when I take out a mortgage?

Depends on the mortgage deal. Most lenders offer you a couple of rates. One rate will be low with an arrangement fee and the other will be higher with no arrangement fees. If you have a small mortgage chances are it is worthwhile choosing a new deal with no arrangement fee and paying a higher interest rate. Your mortgage broker will be able to work this out for you and recommend the correct mortgage interest rate