Leon,
I share your concerns about the stability of the slab and pier.
Malcolm, read this if you can bear it !
See the similarity in construction.
http://www.cloudynights.com/ubbthrea.../o/all/fpart/1
Cloudy Nights also had an amazing thread on a guy who built a very tall concrete pier with infinitely bigger problems than this, but I cant find that link right now.
Rally