Thanks for all replies everyone; I'll consider all advice here very seriously. As far as the value of training is concerned, rest assured that I’m in no doubt how much it counts for, and even if I did do all or part of this myself, I wouldn’t dream of not becoming proficient in air law, navigation, meteorology and so on. My day job involves these things already.
I’m also aware that the kind of lightly loaded airframe I’m considering brings its own challenges. If I were to go it alone, it would be baby steps - I already do this in my professional life with the airframe test programmes I take part in, comparing test results with simulations and predictions at each stage, and only progressing when findings so far indicate that everything is working as expected. I would do this in the same incremental, cautious way - short hops, then ground effect, probably no more than that for a long time.
However, if ever finances permit then I will go the conventional route (which was always my original intention) and divert my self-build ambitions toward an SSDR that I can fly with a properly-obtained licence in my hand. It will be a long time though - I won’t bore people with my circumstances but the last few years have been a series of financial body blows which have eliminated the plans (and savings) that I had in place to do this, and the years ahead are unlikely to be any different. Given that I already work in aircraft design and testing, it seemed natural to look at ways of using my existing skills to make my own dreams of flight a bit more self-sufficient. (It’s no accident I work in this field - the ambition has been a life-long one).
However thanks again for all responses - the supportive ones and the cautionary ones.
F.