How to fix App Cannot Be Downloaded At This Time

Yesterday my iphone 4S started refusing to download apps from the app store, with the error message App Cannot Be Downloaded At This Time.  A quick search of the Internet offered various solutions including

• Turn on Airplane Mode for 10 seconds
• Turn off Wi Fi settings
• Turn off Auto date, set date to one year in the future
• Log out of app store
• Turn the phone off and on again.

I tried them all and they all failed.  Finally I found the simplest of solutions.  All you have to do is turn off your wi-fi router, and then turn it on again.  For some reason (don't ask me why) this resets your app store.  It worked for me!