Hello Ollie,
Thank you for your post.
If you would not like to re-encode the file I would suggest you to use K-frame accuracy for the file editing. For this you could enable the setting in "Tools -> Frame Accuracy (Smart Mode)" menu, but in this case the editing could be not such precise as with frame accuracy.
Here is the useful page regarding the difference with K-frame/frame editing.
With the default settings of frame accuracy Video Splitter re-encodes only few frames at the beginning of the video - the number of frames at the beginning depends on the GOP duration where the Begin Marker is set. And it also re-encode the last frame at the end of the fragment, where the End Marker is set.
See the picture to understand the difference between frame and k-frame editing (made with help of Zond 265) below.
Hope my reply was helpful for you.
Please feel free to ask any more questions.