Building query...

[Len]

It will depend if it is a party wall or a wall soley on your property.
If a party wall then little you can do about it. But I would be very surprised if its actually coming through the wall. If its a party wall it will either be a cavity wall or a solid 9 inch brick wall.
Also if you have a solid floor you wont have any air bricks. Air bricks are only used to vent a timber underfloor.

If this a wall built entirely on your property and I am assuming you are the property owner, then you can make them remove any structure attached to said wall.

[Wazzzer]

Cheers for the advice so far people. Will get some pics this evening so you can have look. It's solely my wall as it's just our extension, then the shed has built against our wall. I'll have a look in case a brick vent has been put in although I don't reckon it has. I'll have a look at the way the roof has been attached. Paint is an idea I've had before as I don't even mind paying for it and doing it myself just to stop the nasty smell

[Len]

When was this extension built? If its in the last 40 odd years it will have had to have been a cavity wall, so I cannot understand how you are getting this smell coming through!

[tweed]

I'm 99% sure it's coming in the roof on that side.

Get some outside pics and inside

[Wazzzer]

It definitely has a cavity wall as we had a hole on our side behind the cupboards which has now been filled. It's baffling me too as the smell shouldn't penetrate the bricks... Like Tweed says their roof fixings are the only likely reason

I did the flat roof not long ago so I could have had a proper look down the cavity etc but I wasn't aware of this problem as the previous occupants didn't smoke in the shed lol

[tweed]

You chase your bricks out and put your felt in the chase then point it back up?

[tweed]

I bet you got a white fascia and a 1" gap away from the brick. Air/smoke will go up and in!

[Len]

Depends if its a cold or warm roof.
Cold roofs have the roof space (between the roof joists) vented so yes that maybe where it comes in

[tweed]

If it was built for a kitchen I should imagine it is vented. If that's the case. Seal it up on that wall. As I originally said get some intubatt and sealant. 

[Egbutt Wash]

Take up smoking yourself.