Well is it law or not?

I was trying to find out a few things today. 1) Whether I am allowed to do gas work in my own house and 2) Whether electrical bonds were necessary across boilers.
Simple questions you would agree but the answers are not simple. The reason I wondered about 1 was that previously I had seen a legal statement saying, "If you carry out gas work for gain.." which implies (quite rightly) that someone doing it as a job must be registered, however, a friend of mine, a gas fitter/plumber, said that now you have to be provably competent to do the work anywhere at any time. A simple search of Google and I found my answers to question 1) Definitely, maybe, sometimes and never and 2) Definitely, sometimes, never. In other words nobody really knows.
Why is this a problem? Well two reasons for me. The first is that depending on the answers, particularly to the first, I might have to shell out £300 for a tradesman which I don't have and secondly it shows the complete irony that in the information age, we lack the ability to distill what is truth from what is opinion. Maybe the truth is not obvious, maybe a document is down to interpretation but what can you do if the people who need to know (like council/gas inspectors/electrical inspectors) don't actually know themselves? How can someone tell me that I must bond a boiler when they don't know the reason just because they have a badge and that's what they've always done. I realised that the problem with sites like Gas Safe which run the gas scheme use words like "Should". You do not use the word "Should" for legal requirements, you use the word "Must". Read the Highway Code (not a bad doc as it happens) and you will see a distinction between you "must obey a police officer's directions" because it is law and "you should use dipped headlights when it is raining" which is not in itself a legal requirement.
If only it was as simple as applying extra weight to the opinions of trained or qualified personnel, however as was clear from my questions, this didn't add much to the quality of the answers. A plumber might have been trained to do something without knowing why so to them, "you must" whereas a pragmatist might completely ignore the law and say, "do it yourself".
At the end of the day, you can try to a certain point to follow the law but if that law is obscured (sometimes deliberately by tradesmen!) I think that is a reasonable defence should legal action be taken against you. As long as you do it all properly of course!