hi all/neil...well 1.5mm aint far off considering your working by hand, sounds to me like drill wander, check your drills are nice and sharp and sounds to me like you need a centre drill, they are cheap like £3.00. place centre drill in punch mark and then start drill.
i have spaced holes using the mill and then measured them and they have been all over the place with a normal drill chuck.
to get a good hole you need to drill undersize and then ream out to size holding the ream in a collet, but this could still be around .20mm off, but as you say, you cant justify spending 100s just to drill a few holes.
a good quality drill press can be used to mill soft stuff by using a compound table, but the drill must be a good one, or a bench top mill for the same money??
redshift is right wirewool or scotchbrite leaves a nice finnish in alloy.
never use sand/glass paper on metal, wet and dry is the one(tungsten abrasive) but the halfords stuff is useless.
personally i use scotchbrite (i get it free from a freinds workshop).
but to polish i use 1500 wet and dry then onto the polishing wheel for a mirror finnish but that is a mission and i dont usually bother.
see ya all...