Thanks very helpful.
So....
1. Buy an external HDD holder from Amazon for £7 or so.
2. Put a new SSD in that.
3. Plug it into the USB port.
4. Ensure the SSD is formatted.
5. Download Linux Mint installing it on the USB SSD.
6. Ensure the machine can boot up from the USB SSD on Linux Mint.
7. Swap the HDD for the SSD.
8. Ensure the machine boots up from the SSD which is now in the same position as the older HDD which should be the boot up drive.
9. Done.
10. If problems the old HDD still has Windows 10 on it, so the machine can work if a problem with Linux Mint on the SSD.

Is that right?