I can't take the credit for finding these - I found a link with loads of expired or non-UK links to all kinds of free stuff.
I've just taken the time to check which are still live and which can be used in the UK.
Thanks to Matty18 for providing a link to a load more; again, there were quite a few expired and even more not for UK. I've sorted through them and added them to the clicky links below.
It is still worth going to the link Matty18 provided as there are a load of add-ons for individual games.
http://www.xbox360achievements.org/forum/showthread.php?t=318598 - thanks Matty18
Please note - while these are free at the time of posting, it is best to check that they are still free before clicking on purchase!