Thanks to Philpub and Rodent especially. Combination of a huge, unique flag and your advice meant that I saw/and was seen at:
Creek Road (between 11km and 7m marks)
Westferry (just before 15m right outside the station)
Westferry (at about 20.5).
Also managed to get to embankment and see (but not be seen) just shy of the 25m marker.
Really useful. If anyones looking to do this in future I can recommend a 5m extendible flag pole (about a fiver from a camping shop) and a 3ft by 5 ft flag. Sits way above most other signs and doesn't annoy anyone in the crowd by blocking their view.