On Mon, 19 Sep 2011 14:21:41 +0100, David Chapman
wrote:

The dimensions of the (cracked) manhole cover, excluding its frame,
are
650mm x 497mm (about 25.5 inches x 19.5 inches).

To prevent problems in the future I'd like to replace it with one made
of something more robust than cast iron,

Can't you cut one in 5mm steel plate and weld a couple of angle irons
on the underside as bracing pieces?