Hello Ciaran,
I'm using the J4 Colette template and am seeing a couple of differences between the demo version on the joomla51 website and the demoinstaller version running on my webserver, these differences only appear in a mobile browser.
Firstly the Homepage - Slideshow seems to create a sidebar on the rhs in which one of the slideshow controls can be seen, also having this module published breaks the sticky header in potrait.
Secondly, in the demo when viewed on a mobile browser in potrait the Homepage-sidecol image appears after the article text. in my version it appears to the side and is therefore very small. I can't work out which Responsive Option would fix this (I have Sidecolumn Positioning set to After component).
Any help or pointers would be greatly appriciated.
Thanks
Emma