Per their website:
Cashback offer valid on accounts opened between 30th April 2018 and 31st August 2018.
The £20 cashback will only apply on the first transaction made in a foreign currency by 30th September 2018.
The £20 cashback will be applied to your credit card retail balance within 90 days of the transaction being made and will appear on your statement as !!!8216;Foreign purchase cashback!!!8217;.
The cashback offer is only valid on one transaction per customer.
The cashback will not affect the minimum payment due. Your minimum payment should be paid on time each month.