In article ,
guv wrote:

I mentioned that there was a manhole cover situated on my drive,
which is just about 2 yards from the drain itself. Lifting that,
reveals the channels themselves are fine from above. There is no
visible blockage, nor any water visible.

Using the word 'drain' implies that you have a U-bend.
U-bends are places where suspended solids can settle
out and accumulate, to eventually cause a blockage.

Before pouring any nasty chemicals down there it might
be useful to stick your arm down the drain and see if
you can feel for what is in the U-bend.

--
Tony Williams.