Few more pics up now. I did mine the dodgy way first aswell. Heres what i did. I used a big centre punch to mark the hole and make a bit of a dint aswell, then i got a 20mm drill bit, the pipe is 22mm o/d, i drilled untill the centre just cut through, then stopped. I then got my big centre punch (pointy end tapers back to bigger than 20mm) and smacked it into the hole, causing the sides to peel over and gives you the full diameter hole without droping shavings into the sump. Good luck and make sure you clean the area properly and weld the back first. The sump is about 3mm thick, with it bolted to the block i wouldnt be to worried about distortion.
Matt