You've centred the rotor haven't you? If not, loosen the mounting bolts, pull and hold the brake lever, then tighten the bolts to get the rotor centred.
I've not touched anything yet. Waiting until the weekend so I have more time for it all to go tits up. But thanks for the tip. I think checking pads is first step.
Maybe
Nope, wheel wasn't spinning so it was definitely one of the pads hitting the disc earlier than the other
Possible. I didn't build it, I just threw it together. Not sure how to check.
I reckon first thing is to pull the wheel and check the pads.