My recent re-aquaintace with the mark 1 shovel, MOT1, and Glacial Till has made me much more cautious about how deep to dig my foul drain trench.
Its not a matter of popping down to fetch red diesel any more .....
I have found these two YT videos very helpful
Osma's guidance (at 4:05)
and this
gentle bit of mickey taking ...
How much shingle needs to be put under the drainage pipe? The guy from Osma says 100mm; is he right? I can't find any guidance that says it must be 100mm.