Size of the cavity

The Building Regulations 2000 (England and Wales) specifically prohibit retrofit cavity insulation in cavities less than 50mm.

The following design conditions have been taken from the BBA joint publication Cavity Insulation of Masonry Walls — Dampness Risks and How to Minimise Them.

The cavity width must be at least 50 mm

When the CIGA inpsector investigated a damp wall which was highlighted by an independent damp surveyor  as being saturated with wet cavity wall insulator he exclaimed that cavity was really narrow but none of this made it into the report.

CIGA tells us you cannot rely on a single measure of cavity width on a property.
In the CIGA"Technician’s guide to best practice Version 2.0 - Issued July 2002  Installing Cavity Wall insulation states"

At least ten cavity width measurements should be made at various places, throughout the building, to give an accurate calculation.

Measuring my cavity 

Using the wall affected by damp, I measured the corner brick to confirm an imperial 110mm depth brick.

Imperial brick on measured at 110mm depth

Chopstick inserted into the airvent until it hit a wall. Cavity Depth marked with sharp knife and  the same depth readings were recorded in all parts of the vent confirming that the tip of the chopstick was hitting a flat surface (i.e wall).

Subtracting the depth of the brick 110mm from the cavity reading....

Gives us a 40mm cavity.

So CWI in a wall with a 40mm cavity is clear violation to building regs , the system designer specification,BBA and CIGA's and guidelines.
The main risk being a much greater risk of water penetration across the cavity to the inner leaf which is exactly what has happened to this wall.
I know that other walls in the house have larger cavities but 1930s houses have considerable variability and the thick imperial brick makes it more likely that there will be narrow cavities.

I have left a message with Building control to see if they have a view on this.