Josie, rather than have us all telling you what it's
not I suggest the best thing to do is for you to watch out for it, try to gather a bit more information and then come back with a more detailed description.
As it's clearly (from your description to date) a bird that doesn't hang around, I'd suggest that each time you see it you concentrate your attention on a particular feature and then make a note of it.
For instance, perhaps on the next visit make a note of the colour of its bill. Then focus on its eye - is there a stripe or mark near it and if so where; above the eye, below it or through it? And what about its tail - is it in proportion to its body (like a Sparrow say) or does it have a particularly long tail (like a Pied Wagtail or Long-tailed Tit - two of the birds already mentioned/discounted). What colour is its head and does it have any markings on it? I'm sure you get the idea.
With fast moving birds it can be difficult to get an overall picture in one's mind of all the identifying characteristics in a single (or even repeated) fleeting viewing(s) and I find that trying to break things down in this way can be hepful in building up a description.
Give it a go and let us know how you get on, as we all now want to know what it is as much as you do
Good Luck
Jeff