Hi Colin,
I can confirm that most VAG diesels suffer with the sinking brake pedal characteristic (having assisted many MOTs over the years). We are a VAG specialist and have seen this characteristic on VAG diesels many many times.

This is a definite characteristic of VAG diesel braking systems as confirmed by the copied and pasted text from the Skoda service bulletin.

Sorry to confirm you wasted time chasing down a characteristic of the system where no fault exists.

Regards,
Jon.