Stupidly I inserted an SD card from my wife's phone without checking which way up it should be. I sort of expected the contacts should be on the under side. Anyway, it wasn't read by the Mint OS.
My question: Is there any way to force eject the card or do I dismantle the MBM2 unit.

In order to put the microSD card with the pins at the wrong direction and have it all the way in, would require a great amount of precision and force to push it. Even so it would be not properly aligned due to the spring click mechanism and the shape of the microSD card that is not exactly symmetric.

There are two options here:

1. The above is true - You'd require to open the chassis and remove the two small Phillips screws to dismantle the front panel to have access to the microSD card's slot.

2. The microSD card has been inserted correctly and is now locked inside with the spring mechanism - Use a long fingernail or a tool to push the microSD further in until you hear a "click" sound, and it should pop back out.
Note: It could be that the Linux OS simply doesn't recognize that card.
You could check at the BIOS menu if it recognizes it ("ESC" on startup to access BIOS menu)

Thanks for you helpful reply. I don't recall using excessive force, The card seemed to enter quite normally. But I didn't expect it would disappear! It doesn't look like there's anything to push on.
I already did a Restart thinking it would sort it out. The BIOS screen came up but all I could see was that the SD reader was set to Auto, so I didn't change anything and exited.
I guess I'll have to unscrew.

You can push the microSD card itself, it's there, just use a tool.
It's a spring click mechanism, pushing it all the way in would result in a click and the microSD should stay all the way inside, locked in; releasing and then pushing it further in should result in another click - at that point the mechanism releases the microSD out of the socket.

It's most probably that the microSD has been properly inserted if no excessive force was applied, I wouldn't recommend unscrewing before making sure pushing the microSD card further inside until hearing the click.

Checking the microSD card in the BIOS is as follows:
1. Enter BIOS clicking "ESC" on startup
2. Navigate to Advanced--> SDIO Configuration
If the microSD card is recognize you should see SDIO device details under mass storage device.
if not, you'll only see SDIO Access mode on [Enabled]

Thanks for all your help Tamir.
After disconnecting everything from the MBM2 I was able to have a good look in the SD slot with a torch. This confirmed there was no sign of a card. So we had a search around. Wife normally finds things for me, but this time she gave up. Later, as a last, daft idea I moved a machine which was standing about 40cm away and there was the card! Some powerful spring in there.
I don't think I'll ever use that slot again.

You're welcome @Captain slog, glad you found your SD card and managed to solve this mystery, well done.