This can sometimes happen when the machine is brand new. This often caused when the rear collector is not seated correctly on the back of the machine. This is because there is a safety cut off switch on the back plate of the machine. If this collector is not seated correctly the safety switch will not be correctly engaged. 

Here are some things to try with the lawn tractor turned off: 

1) Remove the collector from the machine. 

2) Re-attach the collector to the machine ensuring the collector is correctly seated.

3) Sit on the lawn tractor

4) Turn tractor on engage the blades. 

Another reason could be you are not seated on the seat correctly, as there is another safety switch under the seat. So ensure you are seated correctly 

This should resolve the issue and if it does not please do contact the warranty department on 0113 2819219.