Sorry for the delay - thought I had time to dismantle the brakes and measure up the posts yesterday, but life has a habit of getting in the way.
I should have time tomorrow, however, I can only measure diameter of post (accurately) and estimate it's length because the post disappears into a fitting on the rear frame stay. There are however a couple of small holes in the frame fixing point which might help me get quite an accurate estimate. Thread size is anybodies guess and I'm reluctant to attempt to unscrew the post as I think I will damage it - and that's assuming the post is threaded. Will see tomorrow.