You suggesting it would take much less than an hour Freddie? Even without any woodcutting and using 10mm copper, by the time you found the right bits, checked eveything for operating correctly and gas soundness before and after, got the silly clamps on, or half of them because they don't all fit, it's easily an hour for me.
Did one once in 15mm which was a pain to get at , and had very little space behind/above the oven. Took two and a half hours. Shop was selling me at £80 per hour, so £200, plus vat.