As per your table post, I've moved the photos you uploaded to the gallery and added them here so it makes it easier for other community members to see them.
I hope members can assist with your lemon tree. You might find these previous discussions about citrus trees helpful:
Hope that helps.
It looks like sooty mould on the lemon tree @Sporky. You need to act fast.
It's caused by scale insects which are sap suckers. They will kill the tree if untreated. You need to use a pyrethrim-based spray. It suffocates the insects.
Bunnings has a "Perfect Plant Promise" - you can return them for a refund if you are not happy anytime in the first 12 months.
Don't worry. It sounds like you have done everything right!