It sounds like you solved your problem.
But for the benefit of others who might never be getting their I2C interrupt, Check how you left the state of your AA Flag.
If you cleared this flag (LPC_I2C->CONCLR = 1 << 2, or LPC_I2C->CONCLR = 4), then it will not receive a new command, and no interrupt.
Ok, my problem turned out to be a dumb mistake. One part of the mistake I believe is that I did not clear the SI interrupt flag in one part of the interrupt handler (which happened to be the part that was being executed due to another dumb mistake), so the interrupt kept being called which put the chip into an endless loop.
So, maybe one thing to check if the chip seems to hang is to make sure the SI flag is cleared in all possible cases in the I2C interrupt handler.
I'm having a similar problem with a LPC1114 I believe. I've stripped my code down to just initializing the I2C and trying to send some data.
I have a systick timer running and toggling a pin, and when I set LPC_I2C->CONSET = I2CONSET_STA to start transmission, the pin quits toggling which indicates to me that the systick timer quits. The I2C interrupt does not get run either. What can I look for to fix this?
I'm having a similar issue now, after I set the start bit, the interrupt never fires to say its ready. Oddly, it works for a few transfers, then stops working and hangs in I2C_Start on a LPC4300.
Did you ever resolve your issue?