Thanks, that was very helpful. I ended up sending the BTC despite my concern regarding the second strange address and I didn't lose any BTC so... I still don't see that second address anywhere, even under the addresses tab showing all. One of the mysteries of the universe I suppose.
As the user @TheQuin says, it most likely went to a change address. I think there is a way to stop change addresses being used in the Preferences window in the wallet.
However, as long as your coins are still there, they'll just get spent from your change addresses and so can still be used. If you want them back in the original address, you can make a transaction from that change address to the origianl address with the amount that you want to send to it (just if you need everything on the original address) you will have to pay a fee though of course but it'll be very minimal to the 19btc you already have.
