Thin seatpost

well saxon you just need a shim... how much wobbling is there when you put you seat tube in place? if a gap is clearly visable you may need to take it to local bike shop to get a new seat post.

+1

If the post goes in but is a little loose or wants to slip down - find a decent shim (I have shimmed many posts with thin beer can material cut to size)

If the post goes in but is 2 or 3 mm out (there is a 2 or 3 mm gap between the seatpost and seat tube) you need a new post.