-of course daagg, it may just be heat sensitive invisible ink, so anyone who can't get magic markers to work on the missing clues might find the following helpful:
"The writing is rendered visible by heating the paper, either on a radiator, by ironing it, or by placing it in an oven. A 100-watt light bulb is less likely to damage the paper."
After a trudge across fields through the snow I have only just picked up my hardcopy of the Times so have not had the chance to try it yet. I'd be interested to hear if anyone gets it to work.