Strangely enough, I was servicing the car and cleaning the breather system, when I came across the PCV.... not knowing what it was, I asked the question on here with a picture of the solenoid valve. I've since read up on the EVAP system and have a better understanding of what it does and how it works.
After mentioning the fact it was open all the time and not responding to 12v on the bench (it doesn't matter which way you connect it, it's just a coil), I was advised to forget about it and quote "If the car runs fine, if it ain't broken don't fix it.
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Not wanting to go too much off topic with regard to your misfire problems, I've since been told it does matter, and it could cause the petrol tank to collapse (in extreme conditions) if the engine is drawing in fuel vapour from the purge system when the valve is constantly open. It's just passed its MOT test and has been running very well with no fault lights. It's possible the line from the tank to the canister is blocked, but nobody knows for sure.
I don't know if the PCV and associated pipework is causing your problem, but at the very least you need to make sure the breather system / plenum isn't drawing in air from the engine bay, i.e. make sure it's connected (or blanked off) before going to the next step of your fault diagnosis.