So, after fiddling about, including, but not limited to.....copperslip, abrading the shim inside, a different bolt and a few layers of alluminium foil tape round the shim for good measure....no dice.
.
Still slips when i simulate a bumpy road in the garage....may regret that later..
I've measured the internal of the post as far as i can go with big verniers and it looks like its 30.4 one way and 30.5 the other. Its supposed to be 30.3. However, with the foil and the shim being 30.4 to start with im doubting its that.
Also measured the post is 27.21 so pretty spot on.
All i can put it down to is the interaction of the shim and the post..... and the shi and the frame. There's just too many things trying to crush up to get a good tight clamp.
So im going to order a 30.4 seatpost. It won't be as nice, but i agree with
@Imlach , its no use having a post that's barely hanging on in there....especially on long trips.
Im also going to buy a small set of bore gauges as they would ne ideal for this type of thing. Got bigger for engine builds, but nothing that small.
Watch this space.
...think that's all three of us at the same time! 
