Saddle slipping

[summerdays]

I am coming round to the idea of a new seat post... I saw a lovely blue one on a white cube bike tonight.... But maybe they will mark really easily? And will it suit my grey bike?

I've seen the numbers 30.9 on the post so I'm assuming that's the diameter I want.

 

[Yellow Saddle]

I was gonna suggest that to @summerdays, but didn't want to embarrass myself on CC, especially after @Yellow Saddle contributed
I routinely use bits of inner tube as bike fastenings when I don't have the correct fittings

Don't worry, he's fast asleep.