I have opened a discussion a couple weeks ago about draught going on under the floorboards of our 1st floor extension, but I now realised better the extent of the problem.

Apparently there is a very strong draught happening in the cavity wall of our 1st floor extension.
I realised that as I was installing a back socket box into the wall of said extension and seeing as the box is quite deep (45mm) I sadly become more aware of the structure of the walls of our extension; sadly because chiselling off bits and pieces, a good chunk of a freeze block brick broke!

Basically we have, beneath the plaster, a wall of breeze blocks, cavity, red bricks, pebbledash render on the outside. As I crumbled that breeze block when installing the back socket box, I exposed the cavity and could feel right away the amount of draught in the cavity wall, even more so pronounced when we have windy days.

I have drawn a section of the wall, which you can find at the below link.
This wall I am talking about is at the top of a downward street and my feeling is that the wind gets channelled in the street below and therefore our extension is facing a lot of wind.
http://imgur.com/e4vlnQn

On the outside, we have a lead (?) flashing which is at the bottom of the pebbledash render and ends just above the roof of the adjoining one storey extension.
http://imgur.com/Tanb273

My feeling is that the strong wind I talked about earlier seeps through underneath the flashing and it touches directly the red bricks underneath, which in turn let the wind through and into the cavity as they are obviously not perfectly sealed. This is how it looks like if I lift the flashing:
http://imgur.com/3xQ0ooA
(detail)
http://imgur.com/fJlGFlt

I would really appreciated your opinions. I am a novice at improving draught issues and could definitely use some advice concerning how to tackle this situation best, granted that you agree with me that the way the flashing is laid is the issue.

Here are some pics of two roofs of my neighbours; one have an insulation jacked on the wall, the other doesn't but indeed the way the flashing has been laid out is the same. The flashing is indeed laid in a very different manner and my personal view is that in that way there are less chances of the wind going through the wall, as the flashing itself is not acting like a funnel.
http://imgur.com/djpq2Sc
http://imgur.com/FOBY1L8

here the full album if you would like to visualise everything in one view
http://imgur.com/a/yRBEA

Pumped in cavity wall insulation would help reduce draughts unintentionally.

As mentioned above it is important to keep the draughts which will be present in the cavity out of the house.

Try this for advice on how to prevent draughts getting into the floor void
http://readinguk.org/draughtbusters/going-further/first-floor-void/

No point as the insulation would be by passed with the cold air from outside would get into the cavity, it is well nigh impossible to keep draughts out of a cavity.

So insulate the cavity and do EWI.

I'm afraid you are. If you plan to add EWI to a cavity-walled house, you should insulate the cavity first. And if you plan to insulate the cavity then you don't want gales blowing through the insulation.

djh, so if I am understanding your last comment correctly, you are suggesting there is no point in filling up a CW if you do not plan to have EWI and on the same way there is no point in having EWI if your CW is full of draught?

Let me also comment about some points made earlier:
I am aware that CW should have some draughts so to prevent condensation, moulds etc.
But I think I need to perhaps re-describe the situation here and should call the draughts I have as wind. Pure, strong and cold.
To support (or not?) my theory, yesterday morning I woke up to find the floorboard in the room at a temperature of 14.5C. Throughout the night and still in the morning there were strong gales.
This morning I woke up to a same-tempered day, but with no wind. The floorboard temperature was touching 18.6C.

It is therefore obvious to me that true wind find its way in and I think it is much more than it is necessary to prevent condensation and it is coming through in an absolutely uncontrolled way.

Back to how this extension "facade" is built, I am here enclosing some more pictures.
I am quite stubborn that the wind is penetrating from underneath that flashing strip you can see in the pictures, however I do not know what is the best course of action to try and "insulate" that. Spraying insulating foam on top of it? Rendering the exterior wall all the way at the bottom of the roof and laying an horizontal flashing like my neighbours' roofs?

http://imgur.com/a/bUXmp

Truth to be told, I really do not understand the purpose at all of that lead flashing.
The position of its install makes me believe that it was installed to prevent from water to seep through where the one storey extension/flat roof connects to the main building. However, being so vertical as it is, I do not think that leas flashing is actually good for anything at all? Can someone please give his/her opinion on this?
Thank you!