Well...
Here's the video anyway...I've had so much trouble with the I2C on the Arduino, I've given up on it for the duration and written this for the PIC18F4550. 
My other 500 LEDs arrived today so I'll be getting on with the soldering and finalising the 24 x 22 Matrix code on the PIC, then if I can get it reliably working I'll port it to the AVR.
Sorry it's the usual crappy quality but at least you get the idea. 
This is AS1130 #1 providing the clock for #2 and it works very well. IRQs are still a little strange but I've got a handle on it now I think. ]