What about shimming? Have a look out for syntace stuff, very light underrated and would suit the build, both stem and post.
OK, tried shimming the post with a coke can, as read on here as a fairly well used fix, and my first attempt was too good to be true. Cut can, inserted in frame, in went post - sweet.
Sadly on the ride home, the post still slipped. I put this down to not having enough can in the tube, so cut it better a second time around so it took up almost the entire circumference. Will be back on the bike in a few days so will let you know how this went, but it was much more fiddly this time around, and my repeated attempts to get the post in and out, and the shim inside the frame, but not too far has resulted in a very shiny bottom end of what was a very clean and unmarked Thomson post
At least it is below the minimum insert.
Here's a pic of the shim, lets hope it works this time.
On another note, I changed the saddle as the brooks wasn't too comfy. I slipped on a nice and expensive Selle Italia SLR, and at 135g, light as **** too, but man did my rear hurt after the 8 miles to work, so off that has come and on went a saddle I bought off here a couple of days ago. Red corners tie in with the forks and the red bits, has an odd designwith the rails integrated into the body - light though:
Stem has been changed too for a syncros 120mm I had. A huge improvement on the shorter Thomson! Needs decals.