Ah, wonder if that's why Nyra never strayed - we had doors either end of those rooms closed when she was smaller. Weird!
Re: diameter, checked with tape and I have three sizes for different things. In left photo it's not clear but conduit either side of switch is different sizes. Upper is 13mm as it's covering standard power cord, lower is 7mm over flat wire. I also use 16mm for covering a combo of both types elsewhere, and 13mm again on TV co-axial. TBC...