Was it stiff after you'd joined it or from new? Seems odd that you've swapped in a quick link and it's still an issue, unless of course you missed the actual stiff link or joined in the quick link with a pin and it's your joining that's causing the stiffness. Try the side bend again and check that the pin in your chain tool is straight and you've equalised the pin on both sides of the chain.
Was stiff straight after joining it. I think the bit of the chain that it joins onto, and which was where I split it, was slightly bent. The force of pedalling for a couple of k's seems to have loosened it though
More force is always the answer.
Was it stiff after you'd joined it or from new? Seems odd that you've swapped in a quick link and it's still an issue, unless of course you missed the actual stiff link or joined in the quick link with a pin and it's your joining that's causing the stiffness. Try the side bend again and check that the pin in your chain tool is straight and you've equalised the pin on both sides of the chain.