Here's how I solved the problem.
I have been unable to download app updates & new apps for about a week. To solve I
1. logged out of iTunes store
2. Closed all Safari windows
3. Closed all running applications by double tapping home-action key then holding down an app until I got the "-" circle above each. I then tapped the "-" circle on each app. (OBTW this can help save battery by closing background applications). I then tapped home-action key twice
4. I then went to Settings - General then at the bottom performed a RESET. I lost my screen wallpapers & alarm clock settings and had to re-enter my wifi passwords. Photos, contacts & browser history etc remained.
I was then able to download & install apps without any problem.
Best regards. OBTW this my first post even though I'm a know-it-all
Richard
OBTW just after I posted this Apple provided a software update to ios 5.1.1. You may want to update your iOS first & see if it solves the problem. If not, try mine. RM