If I have cloned my system to an externally mounted hard drive using Macrium Reflect, and then remove the hard drive from my laptop and replace it with the hard drive with the clone on it, will the laptop boot without any rescue disks etc.?

If it is exact replica then yes, it will boot as if booting from old hard drive. You can even boot your laptop if that hard drive is connected through usb, just set boot device to USB.

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