Sounds promising Sloth: Hard to tell though if it took 18 months for that iffy plug to corrode so badly,
'Keeping an eye on (suspect) things' is our 'owners motto' of course.....

Sadly e-bay is rife with all kinds of suspects/fakes, especially electronics stuff (as many have found out the hard way....
it seems I have become an expert on counterfeit memory cards like that): Markings/Holograms etc are cloned too,
and apparently regardless of bad feedback stating this !

Many of the (genuine) manufacturers have areas of their web sites telling you what to look for (but often 'too late'):
http://www.ngk-sparkplugs.jp/english/techinfo/fake/

(Yes of course the Counterfeiters also read this data in order to improve their fakes...)

Only times I've seen plugs in that condition is when I've removed them and left them outside for a day or so to rust.. in which case of course they wouldn't have looked like that when removed / Or from an engine that hasn't been run much in a while, particularly if coolant has made it's way into the cylinder or if the engine had none-hydraulic valves and had been stood with exhaust valve open on that cylinder.

If it is a fake NGK it still begs the question how it rusted on the threads... I don't know it spark plug thread repairs are ever done with e.g. helicoils on these engines but such repair could maybe account for rust on threads and different compression?

Well, it has been possibly about 2,000 miles now, and we pulled the offending plug out again on the weekend. It has a bit of the orange tinge going on sadly. No corrosion to speak of on the end of the plug threads or surround, but then it has only been in a month.

The miss at startup is starting to return by the feel of things, along with what worryingly sounds like a bit of a knock. Both the miss and knock also occur on restarting when warm if it has been left a short time. The sound goes away though, leaving only a dull repeating sound which I haven't been able to track down yet. I would think if it were bottom end, it a) wouldn't go away and b) will quite quickly get very loud before and engine-expiring event occurs and it all goes quiet. It has always been a clattery engine, but it seems to be getting worse now.

Either way. I may sling some of the water glass in and then replace the plug a few weeks later and see what happens. If that doesn't stem the plug-tinging and startup miss, I think the worst might have to be assumed, and it will be new engine time.

The engine and Volvo cooling fan have been coping well in this heat, even with the a/c working hard. The knocking and missing at startup haven't been as apparent, though that noise has been replaced with what seems like a bearing somewhere on the ancillary belt getting a bit lary at times.

Today however I've found a little puddle on the ground that appears to have stemmed from the coolant relief pipe on the tank, and few other hoses are leaking. Coolant level has obviously suddenly dropped as a result. The pressure seems far too high.

Definitely sounds not good. If the pressure has got high enough to causes hoses to leak. Did you try water glass?

Well its strange... I think I've been having this pressure problem for a long time now. During the winter, without fail every time after the engine cools down on a cold day, I'd get a leak from somewhere. Never while hot and up to pressure - only as things cool, and I guess contract while pressure still is still trapped in the cooling system. Now its warmer, that behaviour has gone - well until yesterday.

I haven't tried the water glass yet, my sodium silicate seems to have broken down. Will order some more and be trying it soon. Interestingly after driving home, this morning it hasn't spat any more out.

Spoke too soon. Spat out a load at work again. Not sure if its coming out of the expansion pipe though or if the tank has a crack in it.. its wet underneath but the tube isn't really. Will leave it to heat soak when I get home and see if there is any evidence now that I've moved the tube around a bit.

You could always try putting a small container (aerosol can cap?) under the pipe. If it fills up it's coming out of the pipe, if it doesn't, you've got a crack in the tank.

That's not a bad idea.

Typically - got home, left it a few hours. Nothing.

You can bet it will piss all over the car park again tomorrow morning! What with the oil drops (rocker covers... again), and now coolant splotches, its not as if anyone would have a hard time finding where I park.

5 litres of ready made water glass on the way. Hoping it turns up on Friday so I can drain/fill with water and water glass / run to temp for a while / drain / put coolant back in on the weekend.