> does the CTO need to be part of a company that you worked/still work for?
No, there's no difference, it can be any company. Moreover, there are no requirements about whether this company or person needs to be from the UK; it can be any country in the world.
Don't be discouraged from applying because of this! The UK parliament published a report stating that the UK gov failed advertising this visa with too stringent requirements [1]. In reality, you definitely don't have to be a Nobel prize laureate to get this visa, especially if you go through the 'Promising Talent' route.
If you apply, the only thing you lose is £450 and some time. Even if you fail with your application, you can always reapply later, again and again, so there are almost no downsides to trying.
> How did it go?
It went well. :) People compare this visa to O-1, but I think it's better because you are completely free to work on whatever you want and you have virtually the same rights as a permanent resident, with the exception of having no access to public funds (e.g., getting unemployment benefits - you are still entitled to accessing NHS though). Compared to the Tier 2 General visa, the terms are amazing and it's worth all the trouble.
What helped me most with my application were my open source work and conference talks. The hardest part was actually obtaining 2 recommendation letters (I believe nowadays you have to provide 3 of them). You need to find someone in a higher position, like CEO or CTO, who will write a letter for you. I was very lucky with this: I sent a random request to a CTO of a rather well-known open source company and to my surprise they were willing to help me. I'm still astonished by this and I can't overstate how amazing open source communities can be.
Other than that, Immigration Boards [2] helped a lot with technical details - there's a dedicated forum about Tier 1 visas, so I'd recommend reading everything about other people's cases.
Lastly, I do recommend applying on your own (without solicitors). The reason is simple: no solicitor will know about your achievements better than yourself! And the bulk of putting together an application is actually listing your achievements, so getting help in this would be a waste.
Happy to answer any other questions you might have, and good luck with your application.
> does the CTO need to be part of a company that you worked/still work for?
No, there's no difference, it can be any company. Moreover, there are no requirements about whether this company or person needs to be from the UK; it can be any country in the world.