Pretty sure your waste pipes are 40mm, this is undersized.
A vent pipe is a pipe on the system that is taken high up and left open.
If no vent is installed it can create a vacuum on the system and you have trouble with the water not discharging properly. Maybe worth taking to the freeholder if...
Conny, so very sorry to hear your news. I always liked reading your posts, you are definitely one of the good guys . Wishing you and your family the very best