scottrdj,

We also had the problems with checkout pages sticking at step 1, although for us the reason wasn't due to the Royal Mail module, but rather that at the end of some of the checkout PHP files, there were a couple of empty lines in the code. This gave us a "Header already sent out" error, which caused the problem.

There is a module at the ZC site, with which you can output your PHP errors - unfortunately I can't seem to find it now, but I think it was created by Dr. Byte. Have a search for it, it's very useful to track down small errors.