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Very bad engine misfire with symptoms incuding lack of power, vibrations and so on. Mechanics have identified faulty coils but when replacing have not solved problem (assuming plugs are all ok). They cannot seem to get it to fire on all 6 cylinders and no fault codes to suggest options. Had been running fine with good compression although blue smoke on deceleration suggested failed valve guide seals (car had not been driven for quite a few months) . Specialist engine rebuild mech thinks blow-by may be result of misfiring rather than guide seals at this stage. Timing belt replaced (and water pump) by authorised Peugeot service centre less than 5,000km back...is it possible belt tooth has jumped...does that ever happen? Any ideas would be much appreciated.

I wouldn't naturally assume that the plugs are OK, as they could quite easily cause this symptom. Also, oil leaks into the spark plug area could affect the running.
You don't say what year your 406 V6 is but it's important to note that the early (D8) model had a different ignition system to the later D9 model. Later cars had 6 individual coils, and unless all 6 are in good order, there could still be a coil problem.
Regarding the timing belt aspect; did the problem occur immediately the cambelt was replaced? If so, the cam timing could be out. If it was OK when the belt was first changed, it doesn't seem likely to be a belt or timing issue.
Another possibility is injectors. Later 406's did suffer with injector problems.
The best advice would be to start with the basics, so make sure the spark plugs are in good order and there is no oil around the leads. Then check to see if it is firing on all 6. From there it becomes a bit more involved, but checking the cam timing and that the injectors are operating correctly could all be potential causes.
Also, you don't say where in the country you are. If there is a Peugeot specialist nearby, they may be better equipped to trace the problem than a general mechanical shop.

Thanks for that...I'm in NZ for the record and it's a 2002 model...had it for 8 years since 2004 and it's been running very smoothly with some qualifcation. IE: after the cambelt (through Continental Cars) it ran real rough for a while with coils identified as problem....however the car had not been driven or run for 9-10 months so maybe that was part of the reason. Ran smooth-as until cats collapsed into 2 of the 4 exhaust pipes: once removed all was smooth again. Have had peculiar situations.....running typically 406 smooth then became more erratic with roughness. Had it checked a couple of times...ran smooth then starts to get rough/misfire all over again. One consistency is that 'pollution fault' and 'catalytic converter fault' alram always there and really comes into play when missing and coughing away. Will take your advice and make contact with the Peugeot expert suggetsed. Thanks.

It's important to tell the specialist about the cats as this could be a potential cause too. Hopefully, it hasn't sucked any metal back into the engine/valve area. It would pay to do a compression test too.