https://subcard.subway.co.uk/cardholder/JSP/SPHelp.jsp
Customers need to be registered to be entitled to double points, and you need to make at least 2 purchases within 8 days to qualify for double points. If customers continue to make at least 1 purchase every 7 days,double points can be earned.
To re-qualify for double points – as you did before, you will need to make 2 purchases within 8 days to qualify for double points and then make sure that you make at least 1 purchase every 7 days to maintain double points.
simples !