Longest shot ever! Nut for toilet seat.

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So my terrible luck continues. Fancy modern soft close seat comes loose and being a modern toilet the bolt is hidden with access only being from behind the unit. As the toilet is obviously pushed up to a wall and next to a vanity unit, access is to put it mildly bloody impossible. After fumbling/feeling the nut to try and tighten it, i couldnt understand what type of nut it was so decided to remove the nut, have a look and decide the best way forward. After a long impe including lots of swearing and arm ache I get it off. Turns out to be a 'hump back bridge' shape piece that fits into the hole in the porcelain. Then there is a small nut, like a piece of bar with the threaded hole in it. This then I gather snugly fits into the hump. Truly impossible to achieve with the toilet in situ. After realising the nut fits nicely into a 12mm socket, I devise a plan. Rather than removing the whole toilet, I will reduce the amount tightening up of said bolt by putting various big nuts and washers on the thread and just nip the nut up with the trusty 12mm socket.
I got all nuts/washers in place then managed to DROP the bloody nut down the smallest hole at the side of the waste!!!!! After now removing the entire system to check that the nut hadnt somehow miraculesly NOT dissapeared forever, I have found out that it indeed did.

So to the question!!!!!! Anyone now what thread nut size I need?

( I am still gonna space the bolt as I dont see how the nut can get effectively tightened 'into' the hump ).

It helps to get thing off my chest!!!

Cheers all for listening to my rant.|