Dear all,
 
Just to illustrate the views of Graeme and Marlene, an example of how early waders can return to the wintering grounds - especially after a failed breeding attempt. In May this year, several Black-tailed Godwits have been satellite-tagged (implant of a tiny transmitter in the belly) in the Netherlands. As with may Blackwits these days, sadly, one of these lost her young and decided to start moving rightaway: in an amazing non-stop journey she reached Mauretania on June 22nd, covering 4,000 km in 48 hours (averaging 80 km/h!). Another one was also located over Western Sahara on that date. This illustrates the view that early returners are very possible. Of course first year species of other species (such as Whimbrel or Curlew Sandpiper) are known to oversummer, not bothering to risk the migration when they are not ready yet for (or have little chance of) successful breeding, but I don't think this is true for Common Sandpiper. 
Good birding!
 
Bernd