Baxi WM40/4:pilot only lights outside combustion chamber!

[eboracum]

Hello all, I am new and I'm baffled.After many years trouble free service my boiler failed to fire up because the pilot light, despite sparking and having a gas feed at start up,will only ignite outside of the combustion chamber! I discovered this after changing the pilot jet & pilot electrode.As soon as the lit pilot is brought up to its proper position it goes out!This is due to a strong drought inside the combustion chamber so the powered flu is suspected. However the pressure switch operates normally,as the fan is running. Indeed if I block the terminal outside the house for a fleeting moment the boiler starts up normally.
I have examined the flu from outside & found to my great surprise that the air inlet outer cylinder is sealed against the inner exhaust cylinder well before they enter the plenum area.Is this why no air can get in to balance matters? Now I know next to nowt about powered flu technique or design so I am reluctant to mess about with a flu system that has worked fine for 15 years.But the Baxi installation instructions show no such bridging seal between outside the house and the boiler connections.
Any advice from experienced technicians would be much appreciated,please.

 

[twgas]

Top tip........get a RGI.

 

[ianmcd]

you have to be one of the most dangerous arseholes to have used this site, leave it alone and get someone who knows what they are doing before you kill someone

 

[GavThePlumber]

Idiot.

 

[twgas]

Idiot.

Ian isnt that much of an idiot

 

[GavThePlumber]

I should have preceded with Dear op, you are an idiot

 

[twgas]

Ah!

 

[MJGas]

Yorkshire born and interbred. Strong int arm, Thick int head.