Ospreys are starting to arrive now, though sightings away from known breeding areas tend to be 'flyovers' - they are in a hurry to get back to their nests (and some of these birds will be going straight through and on to Scandinavia). Your bird could have been a pale Common Buzzard (they can be as white or whiter than Ospreys).
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Thanks for that, it well might have been a buzzard. Would have been nice to be able to see it more clearly and for a bit longer but never mind :)