A lawn mower comprising a deck (2) mounted on at least one castor wheel assembly (4) characterised in that there is provided a locking mechanism (29, 34) capable of holding the or at least one castor wheel assembly (4) in the direction it is travelling when the locking mechanism (29, 34) is operated. The locking mechanism (29, 34) comprises a braking surface (29) and a braking element (34) such as a brake band or pad which is movable between a first position where the braking surface (29) is rotatable relative to the braking element (34) to allow the castor wheel assembly to swivel about a substantially vertical axis and a second position where the braking element (34) engages and holds the braking surface (29). The locking mechanism is manually operable by an operator standing behind the lawn mower and holding the handle thereof.