Dusty Firebird Combi

[daveb1]

I have a firebird 90 combi about 6 years old. It has been serviced about 4 times over this period. About 3 months before the last service we noticed yellowish dust around the boiler and kitchen. It's been checked for leaky flue, seals etc, no luck.
Gone around it with a gas sniffer and watched it in the dark with a torch still could not see where the dust comes from. It also has a carbon monoxide detector directly over it and it does not move
Anybody come across this before, suspect it may have a split casing that only leaks on start up and expansion seals it when temperature builds up.
Any suggestions appreciated really doing my head in now.

 

[Gas2Air]

Does the flue go through a cavity wall?

 

[daveb1]

Does the flue go through a cavity wall?

Yes the flue goes through the cavity wall balanced type, have checked it out and sealed it on internal and external walls.
Flue pipe is warm but not red hot when boiler is on. Wall is insulated done about 3 years ago. I had thoughts it was the insulation breaking down ,but dont think its that.
Regards Dave