Craig,
for my two bobs worth, nobody has yet mentioned the poxy primary mirror springs that come as standard with the 12" LB. You'll find that if you get the stronger ones from bintel, the main mirror will be rock solid once adjusted.
At the moment, the weak springs will be allowing it to slosh around all over the place, and the locking nuts only prevent it from moving in the one direction - ie they don't prevent movement and sagging caused by weak springs.
I had this problem with mine and it sorted it out.
Hope it helps
