What was their method of checking the rockers previously?
I can't understand how if they checked the rockers already they wouldn't have seen they are broken, they basically snap in half. It sounds to me like they've not checked them at all, they've just fitted a new cambelt, tensioner, idler & waterpump and started it to see what happens, and the rockers have not yet been checked. This is bad news because you now have the cams rotating again with bits of broken rocker floating around in your cambox, the bits of broken rocker will be getting deeper into your engine.
Unless they're spending more time on diagnosis at their own cost, then my advice would be to get the car taken to a specialist before you get in any deeper. All the work they've done so far of re-timing the engine, putting on a new belt etc is going to have to be done again to get back to replace the rockers.