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You may be better removing the existing bolts and putting in some insert nuts and then you can use a standard M4 Screw.
https://www.amazon.co.uk/FLANGED-DRIVE-FLANGE-THREADED-INSERT/dp/B0193Y1QXS/ref=sr_1_3?s=diy&ie=UTF8&qid=1521757228&sr=1-3&keywords=wood+insert+nuts+m4
Thanks. Have established that it's hex barrel nuts I need. Now to find somewhere I can go and by 30 each of a bunch of different sizes in London.