My personal opinion on the best grit and minerals you could possibly use are the following:
SEAWEED, REDSTONE GRIT, PINK MINERALS AND A GOOD GRIT COMBO - MATRIX IS ALSO A VERY GOOD MIXTURE OF GRIT
PICKIN BLOCKS ARE BRILLIANT AND I REALLY RECOMMEND THE PICKING BLOCKS FROM NATURAL
AS FOR THE FIELDING SITUATION
I HAVE SPOKEN TO A VERY GOOD FRIEND OF MINE, AND AN EXCEPTIONAL FLYER HERE IN THE MIDLANDS AND HIS ADVISE IS:
GET SOME SOIL, AND SPRINKLE A LITTLE IN EACH OF THE BOXES
THIS MAY SEEM AS IF YOU ARE DEFEATING THE OBJECT AS THEY ARE STILL PICKIN AT '''DIRTY'' SOIL BUT ATLEAST IF ITS THERE FOR THEM IN THE LOFT AND THEY KNOW ITS THERE THEY WON'T HAVE IT IN THEIR HEADS THEY HAVE TO GO LOOKING FOR IT IN FIELDS THE BIRDS SHOULD THEN BEGIN TO EXCERSISE AROUND HOME
AT THE SAME TIME STILL LEAVE AVAILABLE THE GRIT AND MINERALS FOR THEM TO GET AT.
ONCE THEY START EXCERSISING AROUND HOME AND YOU KNOW FOR SURE THEY ARE NO LONGER FIELDING GRADUALLY WEAN THEM FROM THE SOIL I.E WHEN CLEANING OUT THE LOFT TAKE A LITTLE AWAY AT A TIME.
IF YOU HAVE GRIDS IN YOUR BOXES THEN PLACING THE SOIL UNDER THE GRIDS WOULD BE PERFECT AS THE BIRDS CANT CONSTANTLY PECK AWAY AT THE SOIL AND ALSO THEIR FEET DONT GET MESSY!