Thanks to CP’s rec for the cobalt drill bits and cutting fluid, finishing the job only took 15mins of drilling this morning using the garden bench and my cordless drill.
The amount of material coming away was impressive. I was using cutting fluid, applying minimum pressure and taking my time. Suddenly the whole thing made a bang and retaining plate shot out of the bottom, because the bolt was so that tight.
Thanks to all the advice above, you’ve saved me a lot of time and having to buy a new stem.
Thanks again STW.
*pats on backs all around*