Since buying the carving have noticed that three of the tyres are releasing air very slowly. One in particular is very bad.

Do I need to replace all three or could there be another reason? One of them had 13psi in yesterday!

If the there's still a half decent amount of tread left I would go to a tyre place and see if they can find any punctures that can be fixed. Faulty valves or corroded wheels can also cause this.

Our Fabia Vrs is the same, corrosion on the inside of the rim allows air to escape slowly, a decent tyre outlet should be able to help you with this, they can clean up the corrosion on the rim and then re seal the tyre bead.

Hope this helps

Getting the wheels refurbed is probably a better option in the long-run if you have the cash. Your wheels will look brand spanking new, you will have a choice of colour, and leaks due to rim corrosion will be gone.