Well, we've finally found where the 'ole in the top of the septic tank
is. Doing a bit of garden tidying yesterday, I found a paving slab in the
earth between a tree and the hedge. Lifting that, there was another
paving slab. Then a bit more. Then a brick. Then another paving slab... I
didn't lift that one, because a crack opened up between it and the earth
to one side - a bit of loose earth slid in, and landed with a sploosh.
I've not investigated any further, and have no great desire to. The slabs
are lobbed back in position at the moment.

Generally, not an ideal access setup. /understatement

So - what think you all? I've got a little stack of blocks at the back of
one of the outbuildings. The path of least resistance would appear to be
to lay those into the earth as best as possible, mortaring them together.
Mebbe lay a rough wooden frame underneath the first layer, to ensure they
stay put. A couple of layers, laid "low but wide", should give sufficient
strength without having to worry too much about earthworks. Then a
manhole frame & lid over the top.

There's not going to be any traffic over the top - vehicle or even foot.