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.
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.