I am having extreme difficulty finding steel manhole covers of the
following dimensions:

620 x 620 x 20mm

Single Seal

I've been searching around the 'net for ages to no avail. Note: I only
need the covers, not the frames.

As it's in the garden it doesn't need to be super strong, as long as
it takes my weight or that of the lawnmower then that's fine.

I need to replace two covers, one over the 'solids' and one over the
'water' sections of my septic tank - the existing ones are rusting
away to such an extent that the rust has eaten cracks in the tops, so
much so that that they almost certainly wouldn't take a person's
weight (thankfully they are in my small garden which is hardly used).

Any ideas please?

Thanks