I have never took a time to transfer my I2C library to LPC. I'm still using my old software simulated I2C library with:
i2c_start, i2c_stop, 2c_read, i2c_write and
i2c_recover functions.

It works perfectly ok in a single master -> multiple slave configuration. It's pretty tight and fast.
I know it is not an answer to your question, but maybe you should consider also this option.

i2c_recover function is mandatory to use if you have a watchdog timer or a voltage supervisory circuit used in your application. It prevents I2C bus hang by the slave, if I2C transfer interrupted during the i2c_read (when the slave has forced SDA line to ground).