I was over there in late january/early feb this year, walked in at about 1 on a weekday and got a butterfly from Dan pretty much straight away, I think I was pretty lucky to get straight in though. Then about a week later went in at around 5 wanting something from Bert or Steve but had no luck, they told me to call the next day at around 1 to check if Bert was free then, he was and half an hour later I was getting a panther head from him.
So yeah, theres a good chance you can walk in, the earlier you go the better, something around palm size or smaller and they should be able to fit you in