Dunno if my semi is unusual, but it shares the drains with next door, and I have the privilege of it running back to front under my house. And next door had a blockage. So the drains man wanted to lift the cover in my back garden.
He sorted things ok and it seems the problem was 'wet' botty wipes used for the baby. Tesco ones, made of cloth it says, and apparently don't dissolve as quickly as paper - if at all. He removed a carrier bag full...
No warning on the packaging.