The slots/holes are required to accept brass 'u' pieces which are the supports for the vertical handrails either side of the ladders. There are four supports on each side of the superstructure each requiring two slots. The superstructure is already built as I hadn't considered the railings etc until now. Moral RTFM.
Peter, bummer I was hoping you'd come along with ready solution.
Tiger Tiger, your solution was how I was going to try it unless somebody gave me a better idea.
I can't get into the workshop for the next few days, painting the floor, so the job will remain on hold till then. I'll let you know all how I get on.
Bigh, your idea is also an approach I thought about but I have no grinding wheel (yet!).
Thanks everybody for the help.
Doug
PS Omra85, I tried your method of painting holes but they all come out as the same colourless space.