Whats involved to replace/repair DriveBelt Tensioner Pulley SLK32; at what cost?

I am a UK Amg SLK 32 owner/driver. Mine developed a squeal 10 miles ago and shed the belt all in one piece this evening after my garage had warned me to not drive far (1 mile home!) Seems the Crankshaft pully wears and creates a wobble that moves the belt off the idelr - eventuially it breaks or comes off. The parts come to just over £200 with about 2 hrs labour to pay in addition. Good luck. Apparently this can be a DIY job, if you know how to check that all the idler pulleys are still ok - I don't so it's goping to hospital nect week!