As Vegard says, oil it well beforehand.
I left the bush soak in a bath of oil for a couple of days.
Fit the inner column in the casing without all the gubbins on the top end, slide the inner column down as far as possible, then put (easier said than done) the bush around the inner column and feed it into the outer column whilst pushing the inner shaft back up to its proper position.
If there's a simpler way I'd like to know.
soak the felt bush as has been described, but before you put it in the column as described put it in a vice, using two strong pieces of wood or metal to fully cover it if your vice jaws aren't big enough, wind it up as hard as you can and leave it overnight. Return the next evening take your felt bush which is now the thickness of a piece of Bernard Mathews ham and fit it as described above. You will find it goes as easy as anything and the felt will swell back to size over the next few hours.