MrsER
thers a better way than tippex but its fiddly, another A/B er gave me the tip and you cant tell its been done [ with a little patience] ----- you cut out a tiny square, but it must be from the same page as the crossword, then U-Hu [ the glue that lets you move around bit with the tiny bit of paper] glue it in place , wait a few seconds and write over the square the missing letter, works every time for me and looks much neater than tippex which is a dead give away.
Also you can buy special pens in WHSMITH that let you rub out their ink within 24 hrs and then it becomes permanent..
best of luck. Dee