I am posting this to assist anyone who uses the bit.ly URL shortening service with Tweetily.
First, log into your bit.ly account. I will let them explain why you need to reset your bit.ly API key.
Resetting the key obviously stops Tweetily using your bit.ly, so you need to add the new key to the Tweetily settings. This is not easy to find until you know the secret!
Look for the tick box called Use URL shortner?: This will be ticked. Untick it and tick it again. The extra URL shortner options will then magically appear. Paste your revised bit.ly API key into the box, click update Tweetily Options and you are good to go.