You might want to have a look at this website:
http://www.industrologic.com/products.htm
I do a bit of PLC programming with the Panasonic (old Aromat) PLC, and because it supports all five parts of IEC61131-3, it completely supports structured text programming. I have used ST programming in the past, and though not totally C based, it is close enough that only minor changes to your program would be required.
The name of the current Panasonic PLC programming software is "FPWin", version 5.3 (beta version 6 is available on-line).