We called the guys in to have a look- no problem!
They had to cut a hole in the kitchen ceiling, as the manhole in the bathroom is blocked by the hot water system. No problem, they soon located the issue.
And here's the solution- TADA!
In case you're missing it- it's a funnel and a bit of pipe!
It was suggested to the repairman that he cut a new manhole but that was a ridiculous idea - as if he would have to come back again!
Except.......the next day......
So it turns out that the problem was the funnel- it just wasn't big enough.
You'd never even know they had been there!