http://www.telegraph.co.uk/arts/main.jhtml?xml=/arts/2008/11/22/nosplit/bvtvsatfeat22.xml
I'd say she knew from the start that she would only have one season on the show.
There is life after Doctor Who, as Billie Piper has proved, but Agyeman seems to have had a harder time of it. Following the popular Piper, Agyeman appeared as the Doctor’s sidekick Martha Jones for just one series before she was replaced by Catherine Tate. Although she rejoined them for some episodes and appeared in the Who spin-off Torchwood, rumours started that Jones had been written out because she wasn’t popular enough – a rumour which Agyeman dismisses: “I found out what was actually happening to Martha quite quickly after I started. It was really positive moving on to Torchwood although I know that is not how it was portrayed.”
I'd say she knew from the start that she would only have one season on the show.