Just an update for everyone. I had another shave and it went better, used less pressure and worked up a better lather and I think it shows. My neck has come up in a rash again, but it's half as intense as it was last time. Used the styptic to fix up my first nick. Oh, the pride, the glory...
So I forgot whether it was the Astra or Derby I meant to use, and I used the Derby. I thought it was okay, it felt sharper than the Merkur. Kinda felt like it was dragging across my skin in some places though.
I'm pretty sure I've worked out the problem here though. In the areas where I'm getting irritation, my beard seems to grow somewhat sideways. I'm pretty wary manipulating a blade like this side-on around my throat, so I was a bit tentative and I guess I kinda just scraped my skin. Maintaining the correct angle of the blade is a bit awkward, but practice will help.
Used the Nivea Sensitive Post-Shave Balm too, and that could be why it was better this time. Mum chucked some Body Shop razor rash repair cream in my lap too which smells like, but is mostly aloe vera. Nice thought though.
Any technique tips for shaving sideways guys?