I would guess that is about right. Not all episodes are a week apart in canon time though, several take place the morning after the previous.

So that means Oct's full moon wasn't shown; unless you count the attack on the woman in the park. The full moons would have been Sept. 18th, Oct. 17th, Nov. 16th, and Dec. 15th. There is snow on the ground during the finale and it has been snowing for several shots before Vanessa's possession, so I would guess her possession took place in Nov. or Dec.

According to my quick online search, it doesn't really snow in London until after Christmas due to the heat of the city so they've already fudged facts a bit. =P