Only once, Nick.
My company car, when I was way down the pecking order, was a Austin Maestro 1.6 L.
I had had it for about three months when I could hear a knock when turning. I had a quick look and feel underneath and felt some movement in the anti roll car. I took it in to a Main Dealer, he had it for a day and said there was nothing wrong with it. The knocking sound was back in about 5 miles and I guess he had put some snake oil on it. Twice more I took out back with exactly the same treatment.
So on the fourth occasion I put my car across the dealership entrance -- luckily there was only one main entrance -- there was another at the back and hard of access -- and then went into the Reception area and waited. It was not long before state customers were coming into reception, some saying they could not get in and some saying they could not get out. Eventually the Manager came into reception and asked whose car it was. I told him it was mine and why to was there. he said they would have a look at it, which they did and found that anti roll bar needed changing.
Result. They changed the anti roll bar and I changed the Dealership.
Peter.