The Guardian does have a marker on the map. I explain on the Submit page that it helps to get a street address for Tumblrs because location markers will overlap otherwise.
Right now, the Guardian and Newsflick are the only London Tumblrs without a street address, and Newsflick wins out as the topmost layer. Not sure why, but it might be an alphabetical thing. (The Economist is the other London Tumblr on the map, but I found their mailing address while compiling the data for this project, so it’s not overlapping with anything.)
Luckily the Guardian has their address listed fairly prominently through search, so I’ll make the update this weekend. But if you are submitting a Tumblr, please include your mailing address. I won’t list the street number, but this way your location markers won’t overlap with each other (unless you’re both in the same building… in which case, feel free to give an alternate address).
