Sorry guys only learning here and don't know where my original post went!
here is the solution that worked for me:
email of gift voucher arrives in outlook with a redeem button. You click on redeem now and you are told to download iTunes even though it is already on your computer - this gets you exactly nowhere!
Solution is you hover mouse over the redeem now button in the email and a window will show up which at the end of the link shows the 16 digit redeem code you need.
Write this down and then go and log into your iTunes Account through your iTunes program and when you click on your account icon up at the top to the left of search you will see button redeem - when you click on that you will see enter redeem code - enter the sixteen digits and that should update your account with the gift amount.
It worked for me!