i think your problem is with tesco it seems that they are downloading ok but you being a third party are being restricted which is wrong as you have paid for the download
have you tried tesco support at all?
if you have what did they say?
if you have not looked at the help page find it
here
on the help page there is a piece of free music see if you can download it and then install it on your md
if you can then the problem is tescos end not yours?
please let me know how you get on
hope this helps