First time I went was for the DPF - common fault about the Leon is the Lambda (?) sensor going faulty, and I told them what I thought it was. They said that they would run the regeneration for £200 (or so) and that if that didn't work I'd have to fork out upwards of £400 to have the whole DPF replaced.
When I told them no, their advice for the regeneration was to "rag the t1ts off it" and to drive home (25 miles) at 70mph in no higher than third gear.
Went to Arnold Clark next day, they replaced the faulty sensor and I happily went on my way.
Second time (just a few weeks ago) took it in for engine management light coming on again - no problems with car at all, still drives the same and no limp home mode.
They tried to charge me £82.50 just to run the diagnostic, told me the fault had fixed by itself and that it wasn't a warranty job. They even double checked when questioned and told me it was definitely something I had to pay for. Luckily I was on the phone to Seat Care at the time and they told me that it was a warranty claim (to which the garage backed down). Within 4 miles the light was back on (they told me they took a test drive yet it had same mileage on it when I dropped off and picked up!)
So back to Arnold Clark, and the car is still with them at the moment. Turns out to be a faulty glow plug or something.
A few other people at work have had bad experiences, and one even thinks they are charging customers and then submitting the warranty claims anyway, although I would never suggest that...