Well, rm128 PM'd me with the correct answer the other day, but sportingly left it for others to have a shot.
None having done so, the answer is....
THE MISSILE SILO, which is in Illusion Pot (place that may not be there), with
IS SI (is both ways) after THEM (those people) LE (the French) and OILS (paintings, returned)
Well done, RM, take it away...