https://www.bbc.co.uk/news/articles/c99w9yeed8jo
So far, no missing kings in this car park.
::)
As someone who spent time shovelling that ironstone rubble around 1975, the opinion then was it was ground levelling to fill in the remains of the ditches. It is, incidentally, very hard to dig out with handtools. Most of the interesting parts of the castle were destroyed by the station. At the end of Chalk Lane stands St Peter's church, a fine Romanesque church and site of two Saxon mortar mixers from an early stage of construction.