The RAIB have published an update with some initial findings:
https://www.gov.uk/government/news/d...dated-21092020
The investigation so far shows that the brakes had come on on the third tanker, leading to the leading wheelset to stop rotating; the other three wheelsets continued to turn despite the brakes being on. The leading wheelset developed a significant flat spot, giving it a 'false flange' on the outer edge of the wheels, leading to a this being derailed on a set of points and setting in motion the remainder of the chain of events which unfolded.
Tony