Rusted screw at bottom of cistern?

[roguegraphic]

Hi i've just moved into a flat and noticed that at the back of the toilet there is a metal screw thats at the bottom left of the cistern is like this... (please see pic) I'm sure I've seen these screws being plastic before? I assume the washer must have gone or something?

If it should have a plastic screw/nut/bolt for this where could I get a quality one and what should I ask for?...and is it an easy fix? Thanks for your help

 

[footprints]

This should do it.

http://www.screwfix.com/p/fluidmaster-blanking-off-plug-white/65267

Frankly I would leave it alone though unless it is leaking, as with something that rusted bolted through a ceramic cistern you way well end up busting the cistern trying to remove it.

 

[Nige F]

is it rusted metal or just gunky plastic - the nut should be inside too

 

[Madrab]

Looks like the old overflow that's been blanked off. Run over it with a wire brush, as footprints mentions leave it alone if not leaking and then paint it white.
You may have trouble removing it with all that rust and cause more issue than you fix.