One of these days a race organisor will get in to a lot of trouble because someone is going to have an accident and get in to a serious situation. If they haven't had a proper kit check it will come back to the organisor.
I honestly could not see this happening if it ever went to court.

If the organiser of a race stipulates that certain kit must be carried and all entrants of that race are clearly informed of this then that's as much as the organiser could reasonably be expected to do. In my opinion a kit check is above and beyond what a reasonable court of law might expect.

If an accident was to happen and this accident was deemed to be down to some lack of required equipment how exactly do you envisage an organiser getting into a lot of trouble given that participants have been informed of risks and responsibilities?

I am assuming that race entrants are all adults in the eyes of the Law, if children were involved there would be much more of a Duty of Care from the organiser towards them.