So pleased to hear they are settling, still an absolutely unnecessary occurrence caused by your neighbour.
Hopefully the police will be supportive, bear in mind the animal welfare act may be something they are not familiar and in the main, prosecutions are bought against negligent animal owners, but S4(1) of the Act may well apply:
S4 Unnecessary suffering
(1)A person commits an offence if—
(a)an act of his, or a failure of his to act, causes an animal to suffer,
(b)he knew, or ought reasonably to have known, that the act, or failure to act, would have that effect or be likely to do so,
(c)the animal is a protected animal, and
(d)the suffering is unnecessary.
(Your horses are protected BTY, S2 of the Act refers)
You could also consider something like an animal and equine specialist lawyers association (non lobbyist so will be impartial which is what you need should you feel you'd want a realistic assessment with regards to a private prosecution - but the members have experience and in many cases have equestrian backgrounds so will completely understand the distress caused)
www.animallawyers.co.uk/
link to members here:
www.animallawyers.co.uk/Members%20for%20website.pdf
The thoughtlessness of some people with regards to animal welfare is despairing (I'm thinking the likes of Chinese lanterns etc that cause so much distress and injury) but your neighbours callous response is beyond the pale.
Wishing your mares a good recovery and so sorry for the loss of the foals.