I still went to choir at the time, so it would be before summer 2005
(possibly winter 2004-5).

Wasn't that the Eastbound platform?

They did Eastbound first, then Westbound. The Metronet website has a press
release dated December 2004 to say that the Westbound work would be 9th July
to 31 Dec 2005, but I think the platform was opened a little early and then
the landscaping that they're doing in the photo was done later.

Another possibility of finding a date is the old Victoria Hall Adult
Education centre in Ruislip Manor (big building facing the carpark directly
south of the tube station - http://tinyurl.com/jv5dr) which is there in the
photo, but was flattened earlier this year. Planning permission was granted
for the redevelopment in December 2005 and I know it was flattened by July
2006, but that doesn't narrow it down much does it!

Must've been a weekend though, the carparks at the large shops in South
Ruislip are full. Check the monster tree shadows outside Wickes at
http://tinyurl.com/zocoh !