Hi Griff,
The Heater Bypass Valve will have one pipe going into it, from the back of the engine - and two going out. Depending on the heater control settings in the cabin, it will either route the incoming hot water into the car, for heat, or will bypass the car, and circulate the water directly around the cooling system instead. (Hence it's name). You have one, without question.
If you look at the bulkhead (back of the engine bay) on the drivers side, you'll see two rubber pipes going 'into' the car. Follow the shorter one of these, and it will take you to a black component that some people describe as looking like a 'spaceship'. This device will also have a small vaccum pipe on the top of it.
This is your HBV. Have a good look around for signs of leakage, and test it with the heater set to both hot and cold with the engine running.
In terms of the wet plug, is it wet with oil, or wet with petrol? I have seen in another thread you say the car is throwing out muck from the exhaust, can you describe this more, as it's likely the two symptoms are related?
I appreciate you're waiting for a reader, but you do really need to find the fault codes before you can progress your troubleshooting.
I would recommend when the reader arrives, you read, write down, and then reset all of your codes before doing anything. Then run/drive the car and see what comes back, and work on this basis.
Reason for this, is that by doing the various tests you've done (checking for spark etc) you will have potentially generated codes unrelated to the original fault, and wouldn't want this to become a red-herring for you.
If you get no further following the codes, as a next step, it might be worth a compression test.
Kind regards,
James