Hi 4gottenabout,
The closest match I can find is the 'Imperial eagle' - Aquila helpiaca.
If this is what your Dad saw he was lucky, it's reconed that only 10,000 left in the wild. lots of images out there.
How big is the bird & what colour is it? If it's quite a big one, the nearest I can think is it might be a honey buzzard, as they migrate across East Anglia from Europe.
Hi, well it's not likely but not impossible, although the imperial eagle that I mention with the white markings between the wings is rather rare in the wild and that colour form is from the Spanish species only, they are often bred from captive-bred birds that are kept by falconers and raptor centres.
I wonder if it could be an escapee ?
We need to know a bit more about this bird, 4gottenabout.
Its size, colour and bill shape would be helpful and general impression.
Was it hawk, falcon or buzzard size, otherwise we are all in the dark as there are masses of possibilities, not to mention Tbird's thought of an escapee - which does sound likely, it's true.
You say it was in your garden - on the ground or in a tree?