This is how I fixed this and I think it's a solid repair and probably stronger than the original brackets. When my other one breaks, I will effect the same repair.
The reason they only supply the complete assembly by the way, is probably because the brackets are plastic-welded on.
This is what I used:
4.5mm drill bit
Countersink drill bit
Countersunk M4 Allen bolts (25mm long, stainless steel)
M4 nut (Nyloc in the photo but any nut will do)
Drill carefully through the plastic pillars from the inside of the bracket, all the way through the outside on both halves.
Countersink the top bracket holes on the outside so that the bolts sit flush:
Assemble the two halves of the bracket together and tighten up the nuts on the bolts (not too tight). Make sure the brackets are properly aligned.
Now on the bottom of the bracket, use a Hot Glue Gun to fill the space around the nut with glue. Don't get any glue on the thread as you need to remove the bolt once the glue has set. The glue will now hold the nuts in place in the perfect position for the bolt to be inserted. This makes assembling the bracket onto the bar in the boot (trunk) much easier and also prevents the nut from loosening.
Job done and it looks pretty neat. I actually replaced my stainless steel bolts with some black ones I bought from Amazon and the repair looks like it was original equipment, plus it's considerably stronger than the plastic weld that was there before. These are the bolts I used:
http://www.amazon.co.uk/gp/product/B...?ie=UTF8&psc=1
Hope this helps.