Ok, I've just dismantled it all and there was no sign of bodies - foreign or otherwise under the mainboard.

With a bare mainboard and jumpers pulled I get the same results as before when testing for voltage and resistance.

Voltage on either of 5V pins = about 4.56V
Voltage on the 3V pin = 0.58V (but about 3V on the outer pin)

Resistance between all 3 >1000 ohms
Does it look like I'm going to have to book a holiday in Amersfoort?