Suffolk Punch Petrol Mower Will Not Restart After Shut Off?

I have a Suffolk Punch Petrol lawnmower with a Centura S engine. Numbers seem to be CTR40 and E61000. When I pull the lawn mower cord for the first time it starts no problems. After cutting the lawn, I turn it off to push it round the front of the house and I go to start it again and it is really difficult to pull the cord and it won't start.

If I leave it in the garage overnight it is easy to pull the cord and will start first pull again in the morning?

Any ideas?

By Jon