I took the opportunity to remove the fuel injector rail (while i was changing the rear bank spark plugs) and a small bit of dust/sand fell through the hole into one of the cylinders. How worried should I be about this? Is there much I can do about it?

Secondly I've relieved pressure in the fuel system, but when I take an injector off how much fuel should i expect to be left in the injection system?

try a small hose taped to a vacuum cleaner pipe to remove the sand. been sand you risk scouring the bore. depends on how much went in!

prob 1/3 of the size of a 5 cent piece. I guess its not a great deal. Unless the engine has been degreased before I can't see how you could avoid this sort of thing happening.

Was there a specific reason you pulled the rail off? try the vacuum idea, but yeah...you wont get all of it.