In the situation I described, as the cam followers are the driving force for the problem, they would need to be replaced too. As such, you would need two new camshafts (at many hundreds each) + 24 x £40 worth of cam followers + gaskets etc. I've read that replacing the camshafts on the S54 is an absolute bugger and not a job for an amateur. If this is required, make sure you aren't at the bottom of the mechanic's learning curve! Also, access is really difficult in the Z4M compared with the E46 M3 and might require stripping the entire front of the car in order to slide the very long camshafts out from the top of the engine. This will significantly increase the labour costs. I'd be sure they've made the correct diagnosis and that they know exactly what they're doing before you have the camshafts replaced with unknown second hand items by mechanics who may not have done the job on a Z4M before...