Can you slide a garden hose down between the suction tube (spear) and the casing? If so, you should be able to flush most of the sand out that way. Where we are on the east coast, we don't even use a separate casing. We use a steel casing with screw together sections to get the depth, then once the spear goes down the casing is removed leaving only the spear in the ground. Also where I am it is only sand all the way down for at least 25M, with a thin (300mm) layer of coffee rock about 2M to 3M down, so to get the casing down all I do is shove a garden hose down the centre of the casing and it almost slides down by itself, with a little bit of rotating by a T handle. And yes, if the spear is full of sand it sounds like your screen is buggered, so has to come out anyway.
