Hiya,
i had exactly the same problem, and found that the coil spring had jumped out of the socket within the pull chord assembly - near enough impossible to re-fit yourself - if your mower is still within the 2 year warranty, take it to a mountfield dealer - if not, it will cost you about £30 for a new unit - its easy to replace - just 3 bolts, take the old unit off, and replace with the new unit.
mickey the fish, August 2008