Martin,
I'm well aware of how to contact ASDA or rather how you should be able to contact ASDA. On two consecutive days I laboured through the layers of 'phone menus before being cut off (four times) while conscious that 0845 numbers are not free. When, finally, contact was made with a human being, they were sympathetic but as they could not say when the problem would be solved, not really helpful. The best I got was a (free) 0800 number to call.
Yes, I could get a new sim but it wouldn't have the number that I'm anxious to keep nor come with the loyalty package. ASDA admit it's their problem and a large number of people are affected but they are keeping very quiet about it.
Barry M