velodocuk wrote:

So I thought I'd stick the (100mm) plunger on and see if I got set up
a water wave to dislodge any obstruction. The plunger went in no
bother (to about 6 rod lengths), but when it hit the solid wall I
found I couldn't retract it more than about 10-15cm.

I think you will find some of the large drain clearage companies
will have cameras they can put down to see exactly what has happened,
for a relatively small fee.

We had a complete drain stoppage a year or so ago,
and a local company put down their camera system.
They found there had been a collapse about 10 metres from the opening.
We were given a video of the inside of the drain as a memento,
after they had repaired the damage.

--
Timothy Murphy
e-mail (80k only): tim /at/ birdsnest.maths.tcd.ie
tel: +353-86-2336090, +353-1-2842366
s-mail: School of Mathematics, Trinity College, Dublin 2, Ireland