Thanks all for your suggestions. Turns out it was just the mech hanger blocking the wheel, but in the end I managed to use a method (coincidentally outlined just after) which I came across online somewhere -- thread in a QR rear wheel and use as a lever to align the wheels and straighten the mech hanger. I didn't heat it though, which maybe I should have, as some of the plating seems to have chipped. Hopefully hasn't affected the integrity of it though. Looks straight enough for now but may need some adjustment down the line. Cheers all!