UPDATE: Recently I decided to visit a USC store (USC are owned by Sports Direct) and noticed that when checking the store info online USC had an 0344 number.  I then decided to check Sports Direct stores and low and behold they also appear to have changed to 0344 numbers.

Their Customer Service number is: 0344 245 9230, sadly I cannot check if the old 0870 number still works as I do not have calls to 0870 numbers in my tariff.

Their entire store list can be viewed here