I'll replace the batteries. I did mention in my original post that this is an irregular problem. I.e. it worked yesterday when I switched it to override, today ....... - who knows.(n)
My Honeywell room thermostat is wall mounted and wireless and on occasion if I wish to over-ride or boost the central heating outside of the normal settings the signal doesn't always seem to be picked up by the boiler. Is the 'receiver' within the boiler? Neither I nor my plumber or electrician...