Where can I get a replacement head assemblyforS&J SPJBC28 2in1brush cutter?

I bought a S&J SPJBC28 2 in 1 brush cutter with grass trimmer from Argos a few years ago,bump knob seized so could not remove it, took it to the local garden centre who bodged the repair used a Flymo knob with no bump knob, said best they could do. Any idea where I can get a new head assembly complete, have tried alm manufacturing but no luck