It sounds like a wader to me paulo but which one may be difficult to pin down. a lot of waders have similar markings to those you have described.
Looking at the size you state it sounds like one of the larger waders and one that comes to mind is Oystercatcher. here is one of my images of that bird.
And here is a photo by fisher showing them in flight.
Another one that comes to mind, but a smaller wader is a Turnstone and here is a link to a flight photo of these birds by PMG. these are a little larger than a swallow.
Another large wader (but with a distinctively long beak that I'm sure you would have seen) is a Black-tailed Godwit and here is a flight shot from jdoherty that shows the white on the wings and tail area.
There is also a bird that can look a bit like a House Martin in flight and that is a Curlew Sandpiper but this is a similar size to a swallow.
Do any of these ring any bells?
John