tryhard

Thanks for advice just one sorry one further question.

The waste pipe changes diameter from 6mm on the floor to 4.5mm from the elbows up to the sink. You can see this on the photo. When I cut it back to bond new run along floor and up to sink can I use 4.5mm as this will fit behind the units better. Only 1 single sink, no washing machine/dishwasher etc, fairly light use.