I did a quick search and couldn't find anything much that helped me out with this particular issue.

Recently I began playing my little strat copy and noticed that from roughly the 7th fret onwards the notes on the G string were all playing flat. I adjusted the saddle and eventually had it as far forward as it would go and while there were slight changes, there was nothing significant and the notes were still playing flat.

This is just a problem with the G string. All other strings seem to be fine. Any ideas on what I could do to fix this?

Maybe replace the strings? I've read on here somewhere that old strings will sometimes not intonate properly. Strings are cheap and it may help.
If the notes are flat, the saddle needs to be pulled back away from the string to increase the string length, not pushed forward.

When notes are sharp, it's because the string length is long enough that you're slightly overshooting the correct area where the note's node on the string should match up with the fret. Same with the notes being flat, except the string is too short in that case and you're "undershooting" it.