Indicators won't indicate when headlights on

As above! My indicators to turn left won't flash if I have my headlights on, the right ones are fine, and the left work when the lights aren't on!

Any idea what I might need to look at?!

Got to be bad Earth on that corner, lamps are using the unlit bulb to make circuit

I had the same problem and sure enough it was a bad earth.
What happens is the current finds its way 'backwards' through the left headlight bulb, then to earth through the right headlight bulb. It's a relatively low resistance path so they still work.
With the headlights on, the headlight feed wire is connected to +12V. The stray current from the indicators therefore can't use this as a path to earth, and there isn't enough current to activate the flasher module. The sidelight on that corner will also be dim.

I'd never have thought of that! Thanks! I'll have a play around and see what I can find!