Hi all
I think the underlying issue is other garages are devaluing the diagnostic side of it all by advertising it as diagnostics but all they are doing is code reads.
When they run out of ideas thats when it gets messy
I recently had a Ford Ranger in- customer came direct from another garage who had spent in excess of £1000 , from memory code was something to do with airflow too small, vehicle had AFM- two non genuine then Ford item.New turbo and an Ecu test.
They then where too busy to take another look for two weeks!

I agreed to have an initial look for an hours labour.(£45 plus VAT)
Cut a long story short smoke test showed up split intercooler.........customer then kicked off and refused to pay as all I had done was "plugged your computer in then that smoke thingy"
The other garages only charge £20 !!!!!

This type of story is getting more and more familiar.
Other regular excuses-usually after customer has been charged-
"you have a better computer"
"our computer away for repair"
"you need a different diag socket plug that we dont have"
My personal favorite after I diagnosed a blocked DPF due to glowplug failure-
" Marks insurance is better than ours...if he snaps your glowplug off his insurance will pay"

MARK