Just a quick thankyou to you all for your feedback. It seems there is possibly a collapsed waste pipe which the council main sewer investigations didn't show up. So I now have another problem to resolve via my house ins.
I have a problem with internal plumbing. When upstairs bath/toilet/basin is used water is backing up into downstairs sink. I have just had external drains cleared but they couldn't help with internal waste pipes.
Can anyone offer advice? I am consdiering one shot drain suggested several...