Well, given that there is very little variance in the thickness of what I'm drilling, the laser doesn't actually need to track. It can be fixed to a bracket off the upright part, alongside the drill's chuck - or possibly put a small mirror there and mount the laser elsewhere down a little tube. The tilt of the thing would be small enough to not make any difference to me. Actually thinking about it a half-silvered mirror could lose some of the light into bit of black stuff, couldn't it.
If it did need to track, i like Scotophor's suggestion, but it would be easy just to make an arm that tilted it the right amount as the drill moved up and down.
Joe G, (ah yes, I remember that conversation!) didn't you find that the light from one LED would fill in the shadow created by the other one? Also, you would have had a V shape, rather than a cross, wouldn't you????? Otherwise it seems like a really good idea
Oh and John, this is simply an aid so I can put the pad on the board I'm positioning with my fingers, where the drill is going. Centre punching won't make a difference, nor will any software - but thanks for the response, you always give me something to think about
Anyway, no-one's come up with an answer to my question yet