Well, I tried and failed to fit the spring in-situ - it is almost possible but not quite and after the 5th attempt I gave up!
I tried to compress the spring with nylon ratchet ties and also lengths of solder, but neither was up to the job. the problem is that as you release tension you have got to get the end of the spring securely located in the hole on the pedal and that just did not seem to happen!
In the end I decided to take the pedal out and to be honest it is not that hard a job, but there probably is a way of replacing the spring without removing the pedal!!