I've decided to cut the 20mm piece of Ali I currently have to the correct width for the bed which will leave me a 100mm (ish) wide strip 1000mm long (ish) which I can use for mounting blocks. I'll cut a piece 200mm long and machine a carriage block...

Click image for larger version.  Name: X-axis carriage block.jpg  Views: 1253  Size: 139.9 KB  ID: 14339

Then after attaching the carriages on the master rail I'll move it up and down and clamp the rail ready to drill.

I've made a small drill guide to go in the rail so the drill bit is kept at 90° (or as near as damn)

Click image for larger version.  Name: Jig1.jpg  Views: 1263  Size: 148.6 KB  ID: 14340

Click image for larger version.  Name: Jig2.jpg  Views: 1217  Size: 122.7 KB  ID: 14341
I've put a piece of 10mm Ali plate on the bed and moved my Pillar Drill on to it to drill the holes, a bit like below...

Click image for larger version.  Name: Pillar Drill rig.jpg  Views: 1295  Size: 163.1 KB  ID: 14342

Should make drilling easier (but slower) and I hope not to snap any more drill bits as I snapped one in an end hole on the rail