Hayter mower?

My Aunts Hayter Harrier 41 jump jet ooops!! mower autodrive isn't working and its hard b***y work pushing it ?. She had 'come to you ' service engineer but can't remember name or find receipt otherwise i'd jump on them to re - do service as the autodrive was working before it was serviced. I'm treating as co-incidental fault but have doubts.

Anyone got idea how the autodrive works on this mower. It has autodrive range of 1.5 to 3mph now about .025mph if that.
Haven't looked at it until get replies in case I bugger it up altogether.
Many thanks in anticipation