I've had nothing but good experiences when I've spoken to both Alex and John and especially Martin when I've gone there. I actually asked the very same question as the OP about trying the Wal a few weeks ago. Alex said they had been asked specifically by the owner of the Wal that they try and reduce the amount of people trying it. Since it was on commision, the guys at the Gallery had to try to accomadate what the seller wanted. Not an easy position for Alex to be in I'm sure as he's the one who has to decide who to let try out the bass.And granted he may not have approached the situation with the greatest amount of tact when speaking to the OP.
In fact, I actually just dropped off a bass for a set-up this evening and in the work shop, Martin was speaking to Dave Swift (the bass player for Jools Holland). John was there as well and both were engrossed in conversation with Mr Swift. John noticed me at the door, invited me in and we chatted about the set-up. I wasn't left standing at the door way waiting for the 'big name' to leave.
Fantastic service for everyone including the man off the street in my experience. I don't think I've ever had such greater customer service from any store, guitar or otherwise than when I've visited the Gallery.
Also, Dave Swift is a massive bugger!