I've been playing the piano from a very young age, about 4, and have always found my left hand to be much weaker than my right, perhaps also lazier.

Has anyone got any advice on how to improve my playing in my left hand....?

Thanks!

Just do lots and lots of left hand exercises. Most music has more work for the right hand than the left, and if you're right-handed that hand will be naturally stronger anyway. The only way to strengthen the left hand is to make it work harder.

Something I find very useful is using my stronger hand (RH like you) to teach my weaker hand. If your left hand is weaker than your right hand at scales, practice the scale 5 times if you RH, then switch to your LH, then 4 times before switching, then 3 etc. Eventually your LH catches on to what the RH is doing. This works for nearly any technical challenge.

I think practice is the only way. Have you had a go at Czerny 718?