How can i stop my key light flashing and preventing my car from starting??

Hi Guys,
I am having a mass of problems at the moment. I have a 2002 Saxo VTS. It wouldnt start the other day out of the blue. The key light on the dashboard would flash and the car would only turn over (but wouldnt start).
I have had the aerial replaced and a garage has tried to re-code the key but to no avail. I am now told by the dealership it is my transponder box???
I will be picking up the box this week-end at a charge of £33. Do i have to get the dealership to re-programme it for me or can i do it myself?

Thanks