fwiw, a leaking backbox wouldn't normally cause a misfire imho. It would just sound 'blowey', which I guess is the reason why it was replaced?
If you clear the fault codes, does the 301 code return, or does the fault seem to move to another cylinder?
When were the 8 spark plugs last changed?
If the 301 returns, it could also be a coil pack issue given the age of the car.