Watching the One Show now and they are featuring an item showing that older model android phones don't erase someones personal details and sensitive information like bank details, credit card info etc even when you do a factory reset if someone is selling that phone or giving it to someone else.

They are advising people to take further steps other than a factory reset to make sure their personal details are not left on a phone they are selling or giving away.

Yep. Exactly why I've chucked my old phones into a drawer rather than recycle them.