There's a .zip folder at the bottom of this page
http://press.hse.gov.uk/2016/alton-towers-owners-fined-over-smiler-crash/ that contains the CCTV footage and the expert witness report that details exactly what went wrong and why. Warning, it's 250 pages of the driest reading you've ever seen unless you do this for a living! Unfortunately it's been redacted to remove statements, names and other identifiable/confidential information. So appendices 1 and 2 are blank.
It's crazy that the empty train sat at the top of the batwing for 20 seconds before rolling back into the valley! If the wind had been blowing in the other direction this probably would never have happened.
Interesting to note that no issues were found with the operators or their working practices. (p21, pa. 48) It was 2 different engineers that weren't aware/told of the 5th train being added to the system and it was one of these that cleared the block, clearing this block is what allowed the full train to be sent under "evacuation mode", supposedly to get the riders off safely and get all the trains back into the station. (p5, pa. 10 - p7, pa. 15) He states that the ride and control system itself were robustly designed with an emphasis of "failing to safety". (p. 18, pa. 41) He wasn't happy with the training of engineers, with a lot of it being observational with little to no formal training.(p22, pa. 52)
He lists the errors that lead to the crash from page 25, paragraph 54. It seems 2 engineers weren't told about the 5th train, they didn't check the log book (because there was no official procedure to compel them to), so they were unaware of the 5th train. Having seen 3 trains in the station/brakes and the 4th train at the top of the lift, the engineer was satisfied he knew where all
4 trains were and cleared the block without checking the track because he felt there was no need because he had already accounted for all the trains he was aware of. Not to mention that he was under financial pressure to get the ride back open as quick as possible, with bonuses offered for minimal downtime and clocks in the op cabin to keep track of up/downtime.
All in all, a bit of a ****.