Dont know if you have done this already but I took the complete filler unit out of my 900 tank, yours would be the same.
As someone pointed out the filler cap is secured by 3 allen screws, the other 3
are for show
.
The actually filler neck is held by loads of litle grub screws around the circumference, if memory serves me corect these dont locate in any holes, they are a friction fit. Underneath the filler neck is a rubber overflow hose connected to the filler neck. Once the filler unit is lifted out the black rubber sealing ring, which is visible when fully installed, flips 'inside out'.
you have to carefully put this back in position when refitting, and gently re-attach the overflow hose, then carefully install the whole unit back into the tank without disturbing the seal.
It helps to partially install all the grub screws first, because when tightening them you have to put considerable pressure on the filler neck so the seal stays in place. It took me about 5 go's 'cos the seal kept flipping inside out again.
if you have already done this... then you know....if not have fun
