What are the benefits of doing a diploma or Masters in Journalism, versus developing this as a 'side-career' whilst doing something better paying? (PLEASE read the details)

In my experience, people with a master's degree in journalism tend to go down the academic path (ie. teaching journalism). In this day and age, a journalism degree is something that is harder to justify. What journalism school teaches you is how to write well, quickly, and while under pressure. The degree also teaches you how to gather information quickly and synthesize it in a way that is clear and understandable. Those skills are valuable in any situation, even if you don't end up being a journalist. However, the challenge for journalism schools today is that those skills can be acquired by maintaining a blog and keeping to maintaining it. Over time, your voice emerges, your writing skills improve and your ability to synthesize lots of content gets better. If you have a PhD, focus on writing in that field. Few professional writers went to journalism school but there are a lot of PhDs who do great writing on their topic of study and help the world better understand that topic.

Many people have made a career in journalism without having a degree in journalism. If you want to be a science journalist, however, you might get good benefits from taking a few classes (not necessarily a degree) that teach you about interviewing, finding and vetting sources, fact checking, and (especially!) journalism ethics. Seek out a class that is taught by a person who has had real work experience as a reporter, whether for a magazine or newspaper. I suggest you search for some information about "science journalism" and give some thought to the requirements of that line of work. It's not likely to make you rich, but you might be more satisfied with your working life if you choose the kind of work you really enjoy.

I don't believe a master's degree is a necessity if you lack an undergraduate degree in journalism. A master's degree often is used by mid-career journalists who want to change jobs and want to make connections at the name schools. If you have a Ph.D. in your field, that should be sufficient. As Mindy notes, many journalists do not have journalism degrees. The figure is roughly half of all journalists did not study journalism.

I studied East Asian Languages and Civilizations in college and did not pursue a postgraduate degree, so I can't speak personally to any benefits of an advanced degree in journalism. I have hired people with and without journalism degrees; I cannot recall a case where the degree played any role in my hiring decision.

I'd agree with most of the posters here - Masters in Journalism is not strictly required for many or most of the kinds of jobs you are mentioning. That said, you seem to be in the UK, not the US, and my impression has been that there can be a bit more formality there. Which is to say, the advanced degree might help you quite a bit at the BBC, but not so much at Sci & Tech Trade Magazine Inc.

As a newspaper guy for a little over 10 years, I'd say don't incur the debt for an advanced degree in journalism unless you plan to become a professor. I've known plenty of good reporters without journalism degrees, and 3 out of the 4 I've worked with who had a master's in it are freelancing or doing something else. The 4th is my boss though.