I decided to waste some time improving my Excel skills and plotted the following charts.
Final Viewing figures for Doctor Who from 2005 - 2015:
http://prntscr.com/8lhqks
The black line is the trend-line. It's almost horizontal, but does actually have a slight negative gradient which does show that between Series 1 and 8 there has been a very very slight decrease in the numbers watching.
I've also included the (rather low) figure for Series 9 Episode 1 at the end. Obviously it cannot affect the overall trend seeing as there's only one value, but it will be interesting to see how the trend changes once the Series 9 data is available.
Also, I couldn't find a list of the Overnight values from Series 1 to 8 so I couldn't add them. Does anyone know where I can find such a list?
Here is a graph of the Average viewing figures for Series 1 - 8 complete with Standard Error Bars (derived from the Standard Deviation):
http://prntscr.com/8lhwwy
Again, it shows a slight decrease over the 9 years, but nothing drastic (not yet anyway; Series 9 could be different of course)
Anyway, this was all a result of boredom. I've been off for a good three months and I don't return to Uni for another 2 weeks.

Nevertheless, I don't think it was a pointless exercise. It's always good to improve you Excel skills.
Disclaimer: I'm not a statistician. I didn't even do Statistics at A-level so feel free to point out any flaws you can see.