That makes me feel a bit better... if you're baffled. I've been searching for the "usual suspects amongst non-prnting characters, but can't find anything obvious.

it's something to do with the length of the strings. The strings can be up to 24,330 characters. Only the 25 shortest (<10492 characters) are replaced with CRTL & H. Is there a limit on the number of characers CTRL+H can cope with???