Yeah I tried that - but even after formatting the un-allocated space and not assigning drive letter or anything, the option to extend C drive is greyed out.
I then tried to allocate a drive letter and formatting. Then I see it as a new drive but when I try extend C drive it is still greyed out.
Your free space must be physically next to the C: partition (start and end block positioning for your partitions), im guessing when you cloned you had some hidden partitions that came with and is between the C: partition and the empty partition which windows disk management can't see and isn't showing. I would suggest you redo the macrium clone, in the screen where you see the source/destination drives and you can click/drag partitions, there is an options/settings that shows you the partition start/end points (on the destination drive, don't mess up the source drive if you don't want a bad day). Just add/subtract the number of blocks until the free space is shifted to be next to the C: partition, then when completed you can extend with the windows disk management.
Or you can try using other software, some might be smart enough to reshuffle the partition block positions automatically.