Thanks again everyone for your input and advice.
I've been a silly billy. Driving home, wondering whether I had some shorter bolts or whether I should buy a tap to clean the threads, a thought dawned upon me; straight to the garage, open middle drawer in toolbox, BAM! Cheap tap set, with an m6.1 tap. Wrench is terrible and wouldn't clamp the tap properly, so clamped it with the vice grips instead, and within 5 mins the threads were clean and clear.
Job done.