Completely irresponsible behaviour. At least metal detectorists are expected to have public liability insurance to cover their activities. Has the Portable Antiquities Scheme been made aware of this?
If it can be shown that these people did indeed leave the gates open, then I would hope that the following will happen:
1) This group will lose all their licences ( they have a collective responsibility as an organised group and therefore the individual members should not "expect one of the others" will do things ).
2) The farmers will invoice the group for all the losses they sustained.
3) The railway will invoice the group for the full costs of the repairs to 720 528, the track, the evacuation, the delay repay costs and the delay minutes.
It would probably not hurt if the Portable Antiquities Scheme received a number of inquiries from a lot of "concerned individuals" as to whether or not they have been made aware of the incident. After all, none of us should assume that someone else has flagged it up to them should we?
Points 2) and 3) I would hope will happen anyway even if it transpires the group was operating without insurance in place. That should concentrate a few minds among other such groups nationally and ensure that everyone takes collective care in future.