I wish I knew the trick Steve????
Sometimes they are stupidly tight. I loosen them before removing the belt, it's a 2 person job if they're tight. One holds the tensioner down so the belt won't slip, with his spare hand he holds a breaker bar on the crank pulley bolt to stop the engine turning. The other person swings off a metre long breaker bar until it undoes with an almightly crack or the 10mm allen key rounds out the capscrew head. Then you carefully cut the head off the bolt to release the camgear and buy a new one.

Above ^^^ someone sugested using a tool called an 'impact driver' this is not recomended on camshaft gear bolts as you can very easily punch the camshaft through it's thrust face and into the head, ruining the whole cylinder head.