In most cases; we(all RGIs) are called in as the expert, employed by the home owner to repair the appliance. I'd say very few people would or be willing to try & repair it themselves. I know you get some clueless dafties on here & other forums, but I think they are in the very small minority.
We are trusted to repair it, to be honest & fair to the customer, in the vain hope one would get repeat business & build a good reputation.
If the OP doesn't trust the RGI he employed, then he's correct in trying to employ another. Too be fair to home owners, idiots like Joe-90 & others on forums like him, don't help with the customer/RGI relationships.
Advice to home owners; always employ the services of a bona fide company or sole trader, that is a member of several trade association, has a land-line phone number, sign written van, lives local to you & comes recommended to you by friends & family.