According to KB21648 on blackberry technical solution centre (entitled "Unable to read a review of an application using App World 1.1 when connected using Wi-Fi"), the problem is known and is caused by " This issue is caused by XML parsing failures in Wi-Fi environments."
The suggested work around is to disable WiFi.
Amazing since it all worked fine on the previous version of handheld software that I was running on last week (5.0.0.423). I guess I'll just have to remember to disable Wifi to read reviews.
I hate it when they release new software that breaks things that worked in earlier versions.
Thanks for your help.