The RAIB has just published this report:
https://www.gov.uk/raib-reports/collisi ... th-station
In summary, the Platform was occupied by an HST and the signaller misjudged the remaining space as sufficient to accommodate a 4 car train; the driver unfortunately inferred from his own experience and training that the call-on implied that his platform should be clear as far as the 4-car stop sign, and realised too late that he was entering the same platform as the HST. The signaller had not warned the driver of the unusual nature of this particular movement, but relied on the warning inherent in use of the call-on. It is interesting that the report is not critical of the driver or signaller; the investigators seem to have accepted it as reasonable that the driver interpreted the situation as he did under the circumstances. They have of course made the obvious recommendations on training and communication.
However another of the results of this misunderstanding seems likely to be a further network-wide reduction in the number of platforms & circumstances where permissive working is permitted. Whilst that obviously can't be eliminated because of situations such as the need to couple two trains together, it can sometimes be avoided. Here the signalman could have routed the second train into platform 7 or 8 (as booked) instead, but at the time those lacked a working lift and he was only trying to be helpful to connecting passengers. With the benefit of hindsight, he probably now wishes that he had left things as booked, notwithstanding any inconvenience to passengers with luggage or disability.
A more surprising finding of the report was a (justified) reluctance to break the covers on the emergency releases for the doors. This will no doubt affect a lot more stock than the class 150s.