If you haven't already done so, please check the game disc for any smudges, scratches, or cracks. If any dirt or fingerprints, clean the disc with a nonabrasive cloth. Do not use water or chemicals. Additionally, make sure the portal cable is not bound and completely extended. You will also want to make sure the portal is not near any electronic devices or other portals.
Please try playing offline (disconnect the console from the Internet) to see if the freezing still occurs.
Let us know the results.
Hi there - Thanks for the advice. In actual fact the game disk was brand new and our Wii has never been online (ever, in fact!). However we have now resolved the problem! Having noticed other general postings about clearing space to free up storage blocks, I thought it was high time we did some housekeeping and so we cleared a load of old saved game stuff off the unit. This appears to have fixed the problem and my son has got well passed where it was sticking before. An error message would have been good since these sort of explicit exceptions should really be caught in the code, but hey, it's working fine now!