Macallister ride on mower?

Hi there.
Can anyone help?

I have a macallister JE150/92 ride on mower. When you select forward (or reverse) the mower starts moving without the accelerator being pressed. To stop you have to nudge the transmission into neutral.
Will replacing the main drive belt help?
Replaced the brake shoe and freed up the pistons but it still doesnt hold the machine when a drive gear is selected. Any help would be welcomed.
Thanks :)