On Aug 20, 10:37 pm, Jim K wrote:

oh concrete will set, just keeping it where you want it whilst it does
if raining/water flowing at time - Bin liner - i.e. effectively make
"a bag of concrete"? will mould to contours around pipe and sides of
ditch and not get washed out whilst setting....

perforate the pipe with angle grinder for first couple if yards, lay
pipe on and surround with clean crushed rock, all sat on & wrapped
around in geotextile (to stop silts bunging up holes in pipe)?

That sounds like the plan thanks.
With the geotextile over the top I can cover the whole trench with
soil.
I never liked the idea of an open wet patch under the house.