I've worked for various gas / plumbing companies over the last few years but have decided the time is now right to go it alone. I always do the best job I can and at a fair price.
It's possible that the thermostat was wired in incorrectly and has blown pcb, this would need further investigation to confirm this.
Nothing is out of hours for me, if you have an issue call me.
It is possible to tile over tiles with the right primer and as long add tree original tiles aren't loose. however with heavy porcelain tiles i would always remove the old tiles.
The gas valve could have been tested without replacing the electrodes. don't think there's a con going on though as it could have been a simple mistake if the electrodes were also faulty and the plumber presumed that was the only fault.