I say both and have used Londonderry and Derry in the same sentence before without thinking. Like treacle I'd be more inclined to say the county with the L and the city without. All what you've been raised with isn't it?
Likewise. Although, it says Londonderry on my birth cert and if I want to say that's where I was born, I think it's a bit cheeky of other people to tell me I can't! Like you say, ZepellinBend, controversial!