Thanks for the reply. I don't have a multimeter but could connect the 5V port as an input to a relay board with both led and relay. Connecting the 5V GPIO pin to that setup normaly results in a LED turning on and the relay activating.
With the Pi Plus present, I can see there IS some voltage on the 5V port, but very little as the LED on the relay board barely lights up and the relay doesn't activate. Same thing when connecting to the 5V GPIO pin while the Pi Plus is present.
Interestingly, the 3.3V pins on the GPIO go also amiss when the Pi Plus is present: barely connecting a 3.3V pin to the relay input, which normally does nothing else than activating the relay, instead spread havoc on the GPIOs inputs levels (and turned my home into a Christmas tree as I use GPIO inputs to detect button pushes).
I've now removed the Pi Plus to avoid any damage.
Any thoughts? Frankly I wish I could have purchased a pre-soldered board.