Just asked them to close my account - the URL for the customer service form is https://photobox-mp.custhelp.com/app/ask
When I noticed Photobox in the URL, I checked, and found it's the same company (also paperShaker and Sticky9, who I've not heard of). Just to be on the safe side, I closed my account there as well. At least you can do that online in real time.
I've been a customer of moonpig (apparently) for 15 years, and photobox for at least 8. Their print quality was far superior to tesco as well, but I guess that will have to do now.