I've had a good old maul of the blade to try and get the plastic
> fitting out of what appears a U shaped end to the arm in slots in to.
> Any one shed any light on this eighth wonder of the modern world?
In the U-shaped bit, on the underside IIRC, the plastic insert has a tang / bulgey bit that must be pressed (or lifted can't remember precisely) to allow the thing to slide out easy-peasy. The U-shaoped bit of metal has a square (I think) pressed hole which the tang / bukge 'springs' into to lock it into position and is _extremely_ hard to see until you're shown .... ;)
IIRC some versions (Mine was an S reg, sisters is an 02 reg) an extension bit of the tang sticks out the back of the U, but is still hidden by the arm, which eases the pressing / lifting.