Thanks very much for the prompt reply. String literal (mis)handling is enough to explain my problem. I've heard of PROGMEM but not the 'F macro'... I'll hunt it down. I can probably sort things by moving more into EEPROM.
Copying string (const by definition) literals into RAM is really wasteful and cannot be needed if there is such a thing as the 'F macro': can this default behaviour be changed ?
For convenience, here is the reference to the macro...
http://arduiniana.org/libraries/Flash/