You would be lucky to find that big a field. It's virtually all arable this way. What few grass fields there are tend to have sheep or beef cattle. It's hard enough to find livery with decent winter turn out. Robinson & Hall occasionally advertise land that Bedfordia rent out though.
Juat thought I would say hi as your local to me and good luck finding grazing. Like said above its mostly arable so grazing is so hard to come by. Very lucky to have what I have. If I hear of any I'll pass you on any info