It is essential you (as a syndicate) take out an appropriate employer’s liability insurance policy – for several reasons.
First, your beaters are almost certainly your employees on the day and it is compulsory for you, as their employer, to insure them.
This is a matter of law.
Though the beaters will likely be casual workers, who only work on shoot days and are only paid for the hours they work, under current employment legislation they, as part-time workers, are entitled to much the same benefits as full-time staff.
Casual workers must not be treated less favourably than full-time staff. You are legally obliged to provide £5million cover for full-time employees, and you must also provide similar cover for your casual workers.
In practice, many shoot insurance policies provide insurance cover for beaters and while an accident which occurs in circumstances giving rise to a breach of the Act would, or could, result in the prosecution of the organiser, the insurance should at least take care of civil claims.