I'm trying to source a steel disk of 50mm or slightly greater where I
can weld a nut in the centre and retains an element of rigidity.

I'm currently using:

http://www.screwfix.com/p/deta-galva...ck-of-10/1946j

really its too large a diameter though not a serious issue but is too
thin where it distorts badly whilst welding a nut in the centre.

Are there any off-the-shelf sources I can obtain a couple of these that
are 2-3mm thick? I've run out of ideas!

--
Mike Perkins