Helen,
usually it's more helpful to provide more info such as where you go the number, any postal addresses etc.. then we can look further into it for you..
however I had a few spare mins and did some searching for you...
Asda Mastercard is most likely provided by ASDA Financial Services -
Here They have a Contact Us link at the bottom of the page -
HereThat gives a list of departments which includes Credit Cards -
HereWhich gives the postal address and a contact number of: 0871 522 5888, which when typed in the SNT0870 Database gives: 020 8181 0000
Perhaps you can try that and get back to us?
Thanks.