MG ZT, Rover 25
If you've correctly set the crankshaft marks, the engine will be in safe position (with all 4 pistons in the middle of their stroke) so there will be no interference between piston and valve. Thus you can turn the cam shafts to access the blocked head bolts. Do it by hand, reasonably gently, but you will be working against the valve springs so there will be some resistance. Obviously don't do this if the engine is not in the safe position.