For all those that have been wanting to fit a 30.9 post into a 31.6 seat tube, I have found a cheap answer that doesn't involve cutting up coke cans.......
Purchase a USE aluminium seatpost shim sized 31.6 - 32.4, I bought mine from chain reaction for about a fiver.
There is already a slot cut down the length of this already, simply trim 4mm off one side, effectively widening the slot.
Slip shim into the seat tube, if the gap in the shim has closed up it may be necessary to trim a tiny bit more off, but don't go mad.
Slide your post in and it should be a nice slide fit.
Sit back, admire your work and feel smug.