If it's a pre-made bottle, it should tell you on the label - we've put ours in a strong pan on the stove, but for a party I've had in the slow cooker. Warm enough to warm you up, but not hot.
went to a party thrown by some German friends the other day - and Germans do Gluhwein all winter - and they served it as hot as tea. If it's only warm, then it'll be cool and that doesn't taste very nice at all. (If you serve it in glasses, make sure they can stand the heat.)
Lovely Chris - I had forgotten that. Great finish! The only Black Adder series I didn't like was the last one, mainly because they all got killed in the end.