On Island Park, next to the closed gate that leads into the car park, there's a set of stairs leading up. (And behind them, if you go in the still-open pedestrian entrance to the car park, there's a lift).

Go up. Go diagonally left across the food area (Boswell's Cafe) to the door in the opposite corner by the closed supermarket (old Tescos). Outside that door there's a car park. Head across the car park towards Iceland, crossing the service road. Masonic Hall is on your right - big square building with compasses on the front. It should be 5 mins walk from Island Park, if that.