That was very helpful, I have installed extended headers to make the GPIO pins available above the installed Sense Hat.
However I still think it is an unanswered question as to what GPIO pins are available (unused by the Sense Hat).
According to http://pinout.xyz/pinout/sense_hat
only pins 3 & 5 are in use, but above we see a couple of different ideas:
"Looks to me like it uses gpios 2,3,8,9,10,11,23,24,25 and the ID pins"
It seem that they are using:
Pin 1 for 3V3 power - (so it is still available for use).
Pin 2 & 4 for 5V power - (so it is still available for use).
Pin 6, 9, 14, 20, 25, 30, 34 & 39 for Gnd - (so they are still available for use).
Pin 3 & Pin 5 for i2c - (so still available for other i2c devices to connect, some i2c addresses will have been used by the HAT).
Pins 16, 18 & 22 (no longer available for use).
Pins 19, 21, 23 & 24 as SPI (so 19, 21, 23 & 26 would be available to use as the 2nd SPI on CE1)."
Anyone have the definitive answer - or suggest a way to test each pin. I'm happy to get the mulitmeter on each pin but not sure what it would show if the Sense Hat is connected to a pin but there is no program running to use it, would that pin just pass thru unmolested and show as being available?