I'll keep it brief:
joe-90 - you remind me of someone who made various posts in
Screwfix says how clever he was with all his properties and how he had fitted his own boiler etc. He even went on to post a link to a portfolio of photographs of his work. After numerous faults, building regulation violations etc. were pointed out, he removed them within days. Someone reported him to Building Control . He says he got a CORGI person to check it afterwards but then like you say you pay the money and they do the job. Look closely at the signature on the certificate just in case it says M Mouse.
Paying someone else to do the job - isn't that what Mr van Hoogstraten was accused of?
And if it goes wrong they'll adopt the "Nuremburg" defence - they were only doing what they were told.