I am finally back on my extension again.

i seem to have two options for the pipe which will run under the kitchen floor to carry rainwater from the garage roof

Pea gravel aound the pipe and then 'compressible filler' and some sort of fibreboard as the pipe exits on either side, with lintels in the wall.

Or

weak concrete mix around pipe.

Questions

Does the concrete mix have to be integral with the floor slab

If I go for the easy option what exactly is compressible filler -  and what do I do to cover it - fibreboard screwed to the wall?? And how close can the lintels be to the corner of the kitchen?

I can't find clear guidance on this anywhere whatsoever

Thanks John D

Might be of interest..
http://www.isomass-lbu.co.uk/regulations.html

The idea behind the compressible filler is not to put too much load on the pipe of joints if things move.