Discussion on: Developer/designer?

I'm going to be boring and directly answer a couple of those questions. As someone who has worked on various projects and systems in over a decade, and someone who has the design ability of pile of compost, I'm fairly well placed to answer, I think.

Do I need to spend a load of time learning about UI and UX?

You should spend some time learning about these topics, but they shouldn't be the primary focus. Being aware of what is good practice and current trends (which people get used to and gravitate towards) will help identify problem areas ahead of development. Remember, learning is a continual, iterative process. Focus on your chosen path but also be aware of what links to it, and keep an eye on that too.

Do most people just know what looks good when they are developing a web page?

Not at all. What looks good can be quite subjective.

Apple, who are regularly seen as a golden standard in design, has an awful layout for shipping details on their website (in my opinion).

Amazon, a global leader in e-commerce, have a site so awful in terms of design that if a startup was to go along similar lines, no-one would buy from them. It's far too cluttered and distracting.

Those are my opinions on two market leaders.

My general rule of thumb with development without a design is to ask this: "If this was a product I was trialing ahead of using commercially, would I enjoy the experience?". If you'd struggle with the layout, or think it's messy and wouldn't pay for it, then it's not right.

Just keep pushing on and learning the basics. As you work on more and more projects over time, you'll get a feeling of what works and what doesn't. You'll pull your experiences together from a number of prior sources, and be able to build interfaces with with a good UX fairly easily.

Finally, remember that perfection is the enemy of progress. Don't try to make the UI and UX perfect - just get it built and then improve it over time.