I have just bought an old folding bike, it's a Pannonia Mayfair. I cant get the handlebars to tighten properly, or loosen for that matter. It has a plastic screw thing (see image) which tightens and loosens, but seems to make little difference to how tight the handlebars are. I am not sure how this is supposed to work (or the purpose of it), there appears to be a normal nut below the plastic one, which from its position, looks like it is actually what is holding the handlebars in place. I cannot however get this to move. Has anyone here had any experience with these hand tightening handlebars?