I've got two apple trees in the garden and all of their apples are covered in black spots
What is this? They are quite different from those What causes black spots on my apples?
Any idea how to treat this ?
This looks a lot like Apple scab, a fungal infection with venturia inaequalis that is very common. The fungus is harmless for human consumption and the scabs can be peeled, but the damaged skin can faciliate entry of mold etc., thus negatively influencing storage of the fruit.
The best prevention would have been planting a scab-resistant apple, but this is too late in your case.
In case of an infection, apart from fungicide1, there are a few measures to reduce re-infection:
1 Whether you choose to take the fungicide route is up to you. Your case appears mostly "cosmetic", so if you can live with a few scabs, simply keeping up with removal of infected material could be enough. Keep your tree well-nourished (not simply fertilized, check what it really needs) and strong and it can resist the fungus.