Posted: 14/12/2014 at 09:51
You've not really got enough room for potatoes - one plant would take up all that space.
But, you could grow a few bulbs of garlic and some Early Chantenay or Early Nantes carrots as they grow quickly, you can then harvest them and grow something else (or more carrots) rather than growing the maincrop type carrots which take up more space and take longer to mature.
I'd also grow some mixed salad leaves or Little Gem lettuce - they don't take long and you can cut them as you use them and then sow some more. A few of the early (quicker growing) types of beetroot like Pablo would also fit in there https://www.quickcrop.co.uk/learning/plant/beetroot .
I hope that helps. Good luck