I connected up +5v to the external power pins of the nRF52dk. I see +5 at TP14.
When I set the power switch to 'on', the board does not boot.
Is there something else I should be doing?
It is only the 5V from the USB that is regulated down to 3.3 V through an on-board voltage regulator. The battery and external power supply are not regulated. You can therefore not use 5V on the external power pins.
Allowed voltage level on external supply pins are 1.7 V - 3.6 V.
More information here.