to be attached the main frame of the apparatus further includes a top forward rail 22 and an intermediate rail 23 which is supported upon the beams 15 and 16 and extend outwardly and forwardly therefrom the top rail 22 is upwardly of and forwardly of the intermediate rail 23 each of the rails is connected to the frame by a plurality of arms 24 and 25 which extend from the beam 15 forwardly at spaced positions along the length of the rails so as to hold the rails infixed position forwardly of the beam 15 the apparatus further includes a curved pusher plate 26 which is formed of a semi rigid sheet of a suitable plastics material such as polyethylene which has sufficient rigidity to remain substantially in the curved position illustrated while allowing sufficient flexibility to allow the sheet to be curved and some flexibility to resilient deform in the event of impact with an obstacle the sheet forming the pusher plate 26 is smoothly curved so as to substantially follow the arc of a circle and extends from a substantially vertical portion at the rail 22 past the rail 23 to a rear portion 27 at the beam 16 which is inclined downwardly and rearwardly the sheet 26 is attached to the rails 22 and 23 and to the beam 16 by brackets 28 a rear flap portion 29 is attached to the lower end 27 of the sheet 26 by a hinge 30 which allows the lower flap portion to pivot upwardly and downwardly as indicated by the arrow 31 in a rest position the flap portion 29 is maintained such that.