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That might be the best thing to do! I'll see if we have the tool in our workshop at work, if not I'll find a shop to do it. I think @Howard is right about not wanting to remove any material from the frame in those areas but I'm hoping that even a thread for a smaller bolt, fitted in to the existing holes will be fine for the purposes of stopping a plastic mudguard rattling. It can't do any worse than the cable ties currently in use.
My new Cotic frame has a small hole on the bottom of the wishbone and another one behind the bottom bracket shell, between the chain stays. Both would be really useful for mounting mudguards but surprisingly neither are threaded. What's the easiest way to add a thread, is it something someone who's not particularly handy/DIY inclined could manage or am I better off paying someone to sort it?