They can 't get into the soil though if you have the copper band and /or vaseline can they? They surely can't get into the compost from the top if it means passing through a
nasty layer of vaseline. The eggs may have been in your original potting soil, and if that is the case some slug pellets would do for them when the eggs hatch. Another way of protecting the pot from them if you are desperate is to fill a shallow tray with water and put a concrete or stone block in it, then stand the pot on the block on the island you have made. No slug will pass through a tray of water. Or you could invest in some nematodes which would soon polish them off for you. If you do use pellets make sure the birds can't get at the dead ones.

Slugs cannot swim. Tell that to the ones which have eaten things in my pots which are stood in water and which were clean before they went into the water filled trays.
Try putting a piece of wire mesh over the drainage holes in pts before filling with compost. This is a far better method than the old one of putting crocks as they leave spaces where the molluscs can make a home.

Place a piece of 'J cloth' over the drainage hole and spread coffee grounds on the top of the soil. I have done this with the Hosta plants in pots this year, so far, so good. The Vaseline around the rim of the pots was disgusting to clean off last year!

Thanks for your input folks. Much appreciated.

After years of disappointment, I repotted one of my Hostas and put a decent layer of very fine grit over the top. I haven't had slug damage on it for three years now. My other one in the garden is surrounded by Pulmonaria which seems to put the slugs off because the leaves are quite hairy and scratchy.