Looking for other stuff, I found this option on the EDIT menu (EDIT - TEXT TIDYING - ZAP GREMLINS) but neither the forum here nor my friend Google has managed to explain to me what it does. Grateful for an idiot-proof explanation.
TIA
Richard
Hi, Richard.
From the manual:
Text Tidying ▸ Zap Gremlins Strips Unicode and ASCII control characters from the selection. If you are having difficulties compiling, or have found areas in your text where the cursor seems to get “stuck” when moving through ranges of text, your document may have acquired these invisible control characters from somewhere. This command will strip out all of the Unicode characters falling within the range of #x00 to #x1F, save for the necessary #x09, #x0A, #x0C and #X0D characters, which are used to print spaces, line returns, page breaks and tabs in your text.
Slàinte mhòr.
Thanks, Jo. Stupidly I didn’t even think of looking there. 
RTFM, I suppose.
Barrachd cumhachd don uilinn agad (courtesy of Google translate…)
Richard