Juddering and noise in engine

Went to use my 55 Plate 206 sport. Started and began juddering straight away and every few seconds a loud crunching/tapping noise from the engine, revs drop and come back up. This just keeps repeating. Changed the coil pack and sparks. Checked all fluid levels and fine. Still the same symptoms. Called out roadside assistance and he was not sure but immediately said it is tappets/timing fault so could be timing belt and it has jumped a tooth. Removed timing belt cover and I can flex the belt. Help please, do not want to spend too much and trying to identify source of problem to reduce costs. What do I need to do next?