I’m a complete and utter novice when it comes to all this stuff so forgive me!
This is my first bit of elektron gear. And I want to get rid of a few of the stock sounds, and replace them with some of my own samples from splice.
Any advice/info on how I go about doing that?
you got to use the transfer app 
The factory content cannot be removed. You get 1gb of storage, but the factory content does not use any of that. It does not sample, so you will have to use the transfer app to transfer your own samples into it.
Ah, I’m sure I seen something about a transfer app on a YouTube video actually!
Oh right, that’s cool. I do like most of the stock sounds, it was just if I had to clear more space I’d have deleted them. Is 1GB enough for a good amount of melodic loops & one shots do you reckon?
I think that 1gb is a decent amount, but that is also a subjective thing. It seems that Elektron made the Model:Samples and Digitakt to focus mostly on drums, one shots, etc., as they both only have 1gb storage, and can only use mono samples. You can certainly do more than just drums and bass with them though, and many people do.
FYI the factory content does not occupy any of the 1GB space available for user samples.
read chapter 12 of the manual for a full explanation of how the file system works, how to add samples, and all that stuff
According to this audio file size calculator I found you should be able to get about 1 hour and 34 minutes of uncompressed audio in the model samples - https://www.colincrawley.com/audio-file-size-calculator/
Holy moly that’s impressive.
I just tried this tool and I got 3hr, 8min, 57.868sec. I’m guessing you didn’t choose “1 (Mono)” from the channels dropdown.