Thanks for the tips.
Might've misrepresented my use of the drill bit. I used it to cut out
a 4" dia hole by drilling at half inch spaces around the edge through
pressed engineering brick, so it did take a beating, and I know now
that i was pushing too hard.

............

I needed exactly 6 x 3m lengths of copper pipe B&Q £39.
But a pack of 10 is £40 so I got that instead. I'll leave the four
spare on the garage roof for a few years until they're corroded enough
to bin. ;-)