MM5451 (LED Driver) basics - trouble lighting LEDs
Problem solved- I'd taken a pile of LEDs from a bin of dead components by mistake.

I put the supply voltage up to 12v and used some brand new LED strips and it all works perfectly.

Varied the bits to ensure the pulse is being picked up correctly and sure enough, the lights change according to the pulse.

Sorry to answer my first question, thank you for your help.