On behalf of Liz who sent in an email:-
I've travelled between Sleaford and Lincoln every weekday for over two weeks, as early as 9am and as late as 3pm. Except for once or twice I've seen a bird of prey standing on the grass in the same field. It's definitely larger than a kestrel or a crow and is dark brown with a pale chest.
It looks pretty fit, and once it almost seemed to run a few paces.
On one occasion there was a second bird and maybe a third (although this could have been a female pheasant)
I would love to know what it might be, particularly as I didn't know birds of prey spent time on the ground except maybe for feeding.