What do banks do with fake £1 coins after I pay them in?

The new £1 coin comes in today, and they say that 1 in 30 of the old ones are fake. I have several hundred pounds worth to pay in, which I am going to do before they stop being legal tender. They re all bagged up in the correct amounts so they just weigh the bags when I take them in. If the stats are right then there will be about £50 in fake coins in my hoard. They ve never checked them when I ve paid in before. How do they check them, or do they ever check them? What do they do with the fake ones? I know there are simple tests you can do yourself, but I m not going to check them myself as it will take too long and I would possibly lose out as well.