Arwyn and Paul F are spot-on, as usual.
Do the 30 minute test yourself bofore you go to garage, the human hand is a very sensitive temperature sensor. If one wheel is much hotter than the others you will then have a good idea as to where the fault lies.
You dont need much of a bind to generate enough heat to make the fluid boil, I have seen this on many cars and vans over the years, particularly on ones that are not daily drivers. Your garage wont have the time to do a prolonged test drive to replicate the fault, and simply putting the car up on the lift and feeling for a binding brake when cold is not fool proof. Any input you could offer the garage will help them fix it.
Hope you get it sorted.
