Well after thinking I'd seen one yesterday, I can finally confirm that I have today!! It's a Syrian Woodpecker. They are very similar to the Great Spotted Woodpecker (in looks as well as the calls!), but told apart by the black line from the bill not continuing all the way to the nape, and the black outer tail feather, along with a few other less obvious differences.
Since first visiting Romania in June 2013, I have seen 9 of the 10 species of Woodpecker that breed here. Hopefully it wont be long before I catch up with a Three-toed Woodpecker. The next challenge will then be to photograph them all!