spark plugs

A few months ago my freelander developed a missfire on inspection one of the plugs had a crack down the side of the ceramic .
Fitted a new set of plugs and all was well in the world ( well my bit anyway) untill today when the miss returned GUESS WHAT one of the plugs has again got a crack down the side.
Now once is a pain but twice !
The first set were Britpart (they are only five miles down the road)
the second were a not cheap Bosch set .
So the question is anybody know why this should happen and what should I do to prevent it happening again?
Oh and buying a diesel or setting fire to it arn't high on the list as unemployment has tightened the purse string somwhat

Is the cracking around the centre electrode i.e. in the cylinder. If so the main reason is they are getting too hot. Are they being replaced like for like or have you checked that they are the correct spec for the engine. Detonation will raise the flame temperature and this is most commonly caused by a hot spot due to carbon buildup in the cylinder. Also care is needed when gapping the plug not to put any side load on the centre electrode. Also is the cylinder head thread good as poor contact will prevent heat dissapation.
Cracks on the outside are normally due to either side loads when setting the plugs or to water dripping onto the plug.
Or you could just be unlucky!!!

1.8 or KV6?

Yeah, I was also thinking about it running too hot. Assuming the mixtures not too lean & everything else check out ok, it might be worth running some cooler plugs to see if that helps.

ps Britpart are a disgrace & not worth the bother no matter how cheap, but Bosch should be ok unless they were chinese fakes. Don't laugh, but I recently saw a tv program where these guys were making & selling fake eggs (normal chicken eggs) for their own market - sad or what.

the Britpart were fitted by local 4x4 garage when it was serviced ,the bosch were from Halfords and reccomended fitment.
The crack is on the outside ceramic top to bottom ,
They are a nice biscuit brown so she dosn't look like its to weak
I use a bit of tubing to get the plugs thread started and used a torque wrench to tighten
so they shouldn't get sideways stress my latest thinking is maybe the coil pack rubber plug caps are faulty somehow because it started to miss again today I will have to pull the plugs again tomorrow
Watch this space
Oh shes a 1.8 but I still love her and wont burn her shes not a witch

I'd still try a grade or two cooler plugs.

ps you aint losing any coolant I hope?

No last time I looked she was watertight
but that might be because there is so much snow jammed in every orifice it can't get out(see gaylander in the snow thread)
What I can't understand is why is the ceramic cracked it cant be heat can it? if the outside of the plug was getting that hot wouldnt there be a catastrophic melt down in the cylinder

Out this morning and pulled the plugs out again,on inspection the crack is back .
It looks like No 1&4 are affected so I have swopped the coil and leads for a spare set
fingers crossed we are good to go

I thought you were talking about the ceramic around the centre electrode. That's a strange one - I've never seen or heard of it before.