I've just discovered this thread. Same dit except in one or two respects.
It happened at Cardiff Central station. The police involved were not real police, they were British Transport Police, who, unlike the real police, usually give the impression that they know what they are doing and call you sir/madam not squire, mate or love.
The mother involved did not slip and bump her head, she had a pushchair with a sprog wriggling to get out and she was holding a toddler's hand just after dismounting a train. She let the toddler go to attend to the wriggler and the toddler immediately fell through the gap between the train and the platform. The train started to move slowly but only to adjust its position besides the platform. A member of the public immediately climbed down through the gap and passed the child back up to the waiting arms of other travellers.
When the rescuer got out, the mother, who had stopped screaming but was bawling her eyes out, was effusively thankful but he turned and berated her for her stupidity and lack of responsibility.
That's when BTP, attracted by the screaming and caterwauling, stepped in and, not having witnessed the incident, made assumptions and started getting heavy with the man who had rescued the woman's child, for "harassing" a member of the public, taking her earlier screams and her tears to be a result of the "harassment".
I was there but I have to insist that I am not JD's brother.
It never appeared in the local press or regional TV news and I doubt that the BTP recorded it.
This happened a few days before the date of JD's OP.