The RAIB has now issued Urgent Safety Advice (published 5/11/20):
https://www.gov.uk/government/public...angerous-goods
The investigation found that one of the seals in the relay valve of the 3rd wagon's air braking system was defective and this caused the brakes on this tanker to be applied during the journey. However, it appears that they have been unable to ascertain how and when this relay valve seal had become defective, hence the safety advice published yesterday calls for a review of maintenance processes of these parts.
Tony