Foxy, as a long term member, I cannot understand why you have posted in the style of a newbie who didn't look at the instructions. 

What is the number which doesn't work? Only telling us will get it removed.
Jackson Square Shopping Centre??? I have never been there, or to Bishops Stortford either. That does not mean I can't help you, but a full address is the starting point and I don't understand why you have not posted one, especially as you say you have visited the Next website.
Finally, there is a slim chance that the telephone number is the same as it was when the previous tenants were in the unit Next currently occupies (I assume that it is not new). What shop was there before?
Does the shopping centre have a website?