We have a PRO38CCCSA chainsaw and have lost the ratchet?

i have lost the screw and ratchet? piece that is used to adjust the tension of the chain/bar.
b&q are kindly supplying the screw but dont supply the piece that it goes into (the ratchet? - it sat in the metal part just forward of the sprocket and moved up and down as the screw was turned)
Does anyone know where i would get such a part from?!
thanks
chris