Unusual Birds

Whilst filling up our office bird feeder yesterday, 2 small birds appeared with white caps on their heads. About the size of grey tits, their bodies were silver grey and white and very slim. I have looked through the RSPB website lists of UK birds and cannot find a match. Just wondered if anyone has an idea what they are?
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https://en.m.wikipedia.org/wiki/Snow_bunting
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Hi Pansy unfortunately not, they were smaller and more finch like with a white blaze/cap across their heads, just wished I had my phone with me at the time....
Not a pied wagtail?
Def snow bunting lucky you
Snow Buntings aren't as small as tits or finches.
Where do you live StillLearning?
Hi we live in Thame oxfordshire
Dove, not a wagtail but as dainty where their feathers are black the birds I saw were white and slate grey.
http://www.birdguides.com/bne/details.asp?t=1163183
Unlikely but possible.
Unless my geography's much worse than I think - I doubt you'd see them in Oxfordshire. They're only winter visitors in most of Britain (only resident in the north east of Scotland where they're common on our hills) and in the south, really just the eastern side.
Could they be finches of some kind which have escaped from a private aviary?