Hi Paul, thanks for the reply!
I'll draw up a circuit diagram ASAP.
One thing I thought was that the driver chip has 4 inputs that are taken from the sources of the MOSFETs. In the diagram on the data sheet, there's a resistor between the lower MOSFETs and ground, which would make the inputs from the lower two FETs greater than 0V, but I left that out of my circuit to maximise current through the load (the data sheet diagram is for hysteresis switching - not sure what that means but i'll be just using a square wave input).
I bought another couple of chips to see if I'd broken the first one, and I tried adding small resistors after the lower FETs. All the outputs worked, so i tried removing the resistors again, and found that two of the outputs stopped working. Adding the resistors again didn't fix it so I obviously blew the chip again.
Any ideas why that would happen? Could it be that the source inputs can't handle being driven to 0V?
Like I said I'll draw up a diagram of what I'm doing as soon as I can.
Cheers