i'm being thick

You need a freewheel removal tool.

That is a screw on block and not a cassette.

You stick the freewheel removal tool over the axle and it slides into splines you haven't spotted yet. You then use a large spanner and unscrew it off. If that doesn't work try the removal tool in a vice and use the wheel to lever with.
 
That is not a cassette, but a freewheel.
Most cassette-lockringremovers which go some deeper " into" the cassette will work:
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