As you walk up South St., looks like it's turn right immediately after Marks & Sparks, down the little alley which cuts through towards the multistorey car park exit. Second business unit on the left. So it's just behind Clintons card shop.
Or if you came by car, park up in the multistorey and leave on foot out of the car exit. As you come out onto the road the alleyway is just across to your right. In this case, as you walk down the alley it'd be on your right.