What appeared difficult proved remarkably simple.
With the hitch fully open price out the end cap on the broken arm. With a drift carefully knock out the friction pad. Remove the three torkx bolts and carefully pull off the old arm. There’s a large plastic washer with cut outs for the three bolts . Clean everything. Absolutely no oil or grease! Reassemble the arm including refitting the red lever that links the two arms. Torque up the three torkx bolts.
The new arm comes with a new o ring . This holds the friction pad shaft in position, thus allowing the pad the to move in and out on application of the stabiliser lever. Be very careful not to dislodge the factory fitted o ring. To ensure easy fitting a good does of spit works well. Check it moves correctly within the hitch by moving the lever both ways throughout its full movement range.
Replace the end cap . Done👍