Please can someone explain to me, or point me in the correct destination of where/how I can claim my money back from 123-reg?
I've never used that registrar and have no idea if you can actually get your money back or at least get a credit with them. Some registrars refund and some don't, and some will refund but will keep a small fee.
Login, go to the contact or support page, ask them to drop the name asap. You can't drop it yourself manually, you need tech support to do it for you. Do it quickly because registrars who refund your money have a time limit. It varies, but it's usually a few days. If there's no refund and your cash is wasted, chalk it up to a learning experience. We all must suffer through it

*make sure you don't reg a TM name, they're just trouble waiting to happen.
*review registrars, and make sure you're getting a good one. Many sites (like Namepros) offer registrar reviews. I'm not saying there's anything wrong with 123-reg, I've never heard of them. But in the domain game, you want a registrar that makes things easy, since you'll be buying/trading/pushing/transfering/dropping names often enough, so make sure your registrar is rated well for useability and tech support, and security of your names. And a note that many domainers will not buy a name that is at an unknown registrar; they'll let the buy go, or will insist the seller first transfer the name to another registrar themselves, and then push the name over. I haven't looked into 123 and they may be just fine. But check around yourself, to make sure you're with the registrar you want. Registrar is very important, not an afterthought.
GL