I'm not an athlete or considered 'fit' but my heart rate whilst sleeping goes as low as 43bpm is this normal?

May be Normal: Do you take any pills to lower your heart rate? If this causes any symptoms? Fluttering or palpitation? If you don't take any medications or does not cause any problems, may be okay but its good to see a doctor if this persists.

Slow heart rate: It is common and normal for heart rate to slow during sleep. Unless there are other things to make you worry about a slow heart rate (such as fatigue or shortness of breath doing exercise, or dizziness or fainting) I would not worry about this.

Be seen: It could be normal, but it would be wise to be checked and probably have an EKG. There are people who just have a slow heart rate, known as bradycardia, but there are also people who have a heart block that may need treatment. An EKG can distinguish between the two.