Thank you all for your help!
Below is what I sent to Brent Council last night:
Fingers crossed!
Dear London Borough of Brent,
I challenge the liability for PCN BT19586168 and BT19586168A on the ground that the alleged contravention did not occur. After parking the vehicles I would have walked north-west towards my house at number 40, and there was no parking suspension sign attached to the regulatory sign for the bay just outside my house. Having seen and knowing that sign, I was entitled to rely on it to park my vehicles.
The obligation of the council is to make clear arrangements for adequate information to be provided about any parking restriction (and to be clearly visible and noticeable) to traffic users. This was clearly not the case in this instance for two reasons:
1) there was only one sign at No.32 St Johns Avenue for a long stretch of restriction covering houses 32-36 with no additional signs provided at the other end
2) the only sign provided was hidden from view inside a tree as shown in the attached pictures, one of which taken by the issuing officer her/himself. This made it very difficult to notice that there was a parking restriction in force on both sides of the street with the restriction on the other side clearly visible by the 2 (!) signs posted near the works taking place.
I trust you will be able to cancel the PCN on the grounds given above.
Yours faithfully,
Wolfgang Frese