Have seen a sparrowhawk pecking over the last remains of a wild bird (?wood pigeon?) in our old garden which is about 500m away (as the hawk flies) from our current garden. It is a concern when the new hens free range around the garden but what can you do? We let them free range at weekends but keep them in the run during the working weekdays. My risk assessment is that fully grown ex-bats are likely to be too big for a sparrowhawk and that the garden is adequately protected from daytime attack from a fox, but I'm inexperienced so could be wrong with this. Its a bit like having teenage daughters - balancing being overprotective vs letting them socially free-range after assessing the risk (my 2 are now young adults and seem to have survived).
Just some thoughts rather than a proper answer so don't know useful this post is.
Hope it doesn't happen again to yours.