We've always had problems with drains blocking up ever since I've been here.  I thought I had found and cured the problem (a collapsed pipe under where I used to park my lorry) but a couple of days ago there was that familiar pooey smell outside.  This time, the block was actually visible in one of the manholes.

There is a half pipe running front to back which carries the foul from the main soil stack. There is then an angled half pipe entering from the side which is a bit higher up.  This is fed from the gulley that takes the washing machine waste.

What seems to be happening is that the main front to rear half pipe is covered in a thick build up of what my neighbour described as 'shale' (sh*tty limescale) and if a particularly large lump of solid waste comes along, it gets wedged between the bottom of the half pipe it is in, and the side entry from the gulley.  This then backs up everything behind it.

I've tried scraping this stuff out with a spade, but it is very solidly in there.  Is there any industrial type descaler stuff I can put in there to clean it out, or am I gonna have to fork out for someone to come and jet them out?

I've had this stuff build up in the trap in the loo and left it soaking in neat caustic and it did nothing. In the end I chiselled it out and it came away like stone. Unless it is easily accessible I'd get a drain cleaning firm in, they are used to dealing with this and have the gear to clean it out thoroughly.

as an aside

if a drain is shared by 2 or more properties they are now the  responsibility off your local water company