This picture is something that has recently popped up in my dad's garden and we are unsure of what it is. Any ideas?

It could be Johnsongrass (Sorghum halepense)

It could be

I'd venture to say that whatever it is you do not want it. Start now to eradicate it. Looks very invasive

Oh no, RHIZOMES! Whatever it is, you definitely want to fully remove it. Those below ground tuber-like horizontal roots called Rhizomes will be your worst nightmare if that is something you want to eradicate. The rhizomes spread underground searching for new property, all the while popping up new leaflets/ stems along the way.