Full story here http://www.headforpoints.com/2014/07...s-credit-card/ but to quote

When you have earned a voucher, it will appear in your avios.com account.

From mid-2014 you need to redeem the voucher online – you can no longer redeem by phone

There does NOT need to be Avios availability in the orginal booking class, only the upgraded booking class. Our guinea pig proved this by booking two business class seats on a European flight where no economy seats were available.

(This is an important point. Upgrade vouchers often require availability to be showing in both the base and upgraded categories. This does NOT happen here. In effect, what the voucher effectively does is let you book an Avios reward seat for the price of a reward seat one class lower.)

The voucher lets you upgrade (or get a discount on) one return flight or two one-way flights. If you want to upgrade two one-way flights, they MUST be done in the same booking with both passengers travelling together. You CANNOT upgrade one one-way flight now and another flight next month as the voucher expires as soon as it is used once.

Whilst the voucher is only valid for 12 months, you only need to BOOK within this period. Our guinea pig proved this by booking for mid 2015, way after the expiry date of his voucher.