Majestic birds, aren't they? We often see him (or her) circling over the field behind our house in Mandeville Road, just up the road from your sighting. I gather the nearest reported breeding pair are based at Knebworth. They have gradually spread their wings, so to speak, since the species were reintroduced in the Chilterns in the 90s. They have no natural predators so are top of the food chain.