What I found with plumbing is that you really pay for the fact that they can do it a darn sight quicker than you rather than necessarily doing it any better. I.e. Joe Plumber might fit a shower mixer in 1 hr, whereas I might start the job thinking it'll take 1 hr, but then xyz will go wrong and I won't have thought of this that or the other, then combined with a trip to Screwfix I'm suddenly looking at 3 hrs.
Before a plumbing job, I tend to trawl the above resources and find out THE proper way to do the job. No short cuts, no cheap parts that a pro plumber might (stress might) employ.
Despite having no formal training, I trust myself far more with basic-moderate plumbing jobs (not including gas-related) than I would any paid pro.