Android Marketplace & (insert application name here) could not be downloaded due to an error

Just recently I ran into this problem on some of my android devices where by they would persistently tell me
(insert application name here) could not be downloaded due to an error
when ever I tried to download new or update old applications
I then spent many hours going thorough forums / bug reports (Google bug reports / Google groups /XDA developers forum to name but a few)
This gave me many things to try, suggestions ranged from making new folders and settings their permissions via the terminal to wiping the dravikcach to making a new Google account and so it went on and on.

In the end however I found a solution that worked for me
In simple this was just a case of force closing every (non system critical) Google supplied app on the device and clearing their cache and data, then finally clearing the data for the "settings" app, (note at this point the settings app that you were just using to do the clean up will close, this is normal behaviour), then shutdown and restart the device (note reboot is not enough)
Finally restart the device and go back to the market, accept the T&Cs again then you should be able to download thing normaly
This worked for me across several devices