Auto drive mower?

I am absolutely knackered my aunts mower..
.Hayter Harrier 4.. autodrive is not working.
Its 3yrs old and the autodrive is not functioning the range was 1.3-3mph now about .5 mph if that, cuts fine, but has to be pushed.
Ir was serviced last year but cannot afford to have yearly service at their prices and wasn't very good at selfpropelling then when it was collected, but service centre said it was fine when they tested it.
I'm now wondering if they created a slow active fault hoping for a repair job.
What could the cause of its failure be?.
Many thanks in anticipation.