For anyone who needs a relatively cheap fix, I found that using a 150mm long, 10mm wide bit of steel, some JB Weld and large pop-rivets provides a sturdy fix.
Looks awful, but I'm well aware that a lot of owners are running these cars as 'bargain basement' machines - and it provides a cost-effective solution.
Drill 4 matching holes along the strip and area around the failed mounting (one of the middle holes is where the ball joint itself fits).
Use JB Weld on the back of the strip when riveting in place, and then once fixed apply around the edges of the strip to take the loading when in use.
Has worked for me opening the boot around 5 times a day (it's my load-lugger) and still holding fine several months later