Any ideas as to cause of P0306 and P0300 code. i have replaced plugs just about 500 miles ago. Think it could be the wires? Thanks for any help.

 

What brand of plug did you install, how many miles on the engine, what octane gas do you use????
306, number 6 misfire is the most common of the rover misfires and yes, a set of 8 mm Magnacor wires will be the fix as long as you are not showing a #6 coil fault also.