Here the colonies, my 99 C280 has a V6 with twin spark, that is two spark plugs per cylinder. They are attached to coil packs, which you need a special tool to remove and the only place I found it is at a dealer. The coil packs are plastic and if you break one big US $$, so a lot more Pound Sterling. Other then that a spark plug wrench is needed. The Mercedes plugs in my car are supposed to be good for 100K miles, sometimes a controversial subject. Your mechanic's remark about waiting for a miss fire is not good advice, as a misfiring plug could burnt the piston etc. I've had my car 4 years (starmarked preowned) it has 49K plus miles and runs very well. I have no intention of changing the plugs until 75K miles comes around. However, lack of use can carbon up the plugs. I would take the car out on the motorway, (watch out for the camera they use in the UK for speeders) and drive it as legally fast as possible and use gear 4 or sport driving mode as they call it here, a couple hour drive would be the best, and don't worry about the tranny, all you will do is burn a bit more petrol. Try this and you will burn off the carbon that might or might not have built up. If the car is running good, leave the plugs alone. Or is your are mechanical inclined buy the tool to remove the coil packs and replace the plugs. I would recommend Bosch Platiniim Plus 4. They are available here, don't know is the Plus 4 plug is available in the UK? Here in the UK a Mercedes shops charges $400 US and up for a tune up and plugs at the dealer I go to are extra. I go to a private shop and he charges the cost of the plugs plus $50 US labor, The plugs are around $8.00 per if the shop gets them. If you bring your own maybe under $5.00 US, but the labor is jacked up to $60.00 per hour, to make up for the profit he lost not supplying the plugs.

Cheerio!

 

thanks for the info 280 wizard
I managed to get one of the plugs out [front right hand - tight for space but the easiest of the lot to get at without removing anything else] and it appeared to be in perfect condition- burning nice and cleanly. I think your comment about leaving them for 75k is a sensible compromise and barring anything drastic in the meantime i think i shall do the same.
ta ta for now