I purchased a "Basil Mara XXL Double Pannier Bag"

Today, with a small load, they seemed to explode off and fly over the road. It appears that the stitching was meagre, which held the two straps to the panniers.
I could buy some belt strap material and use thick Nylon thread to repair and reinforce them, but doing this is time intensive I have found; I would prefer to attempt to return them and get new ones.
What advice can you give me on buying these saddled bags? Are some brands more sturdy than others or is it impossible to tell from brand alone? Street level shops I visited have a poor selection in London, these double panniers appear to be quite a rare breed - any good places to look? Recommendations?
edit
I used to use separate individual panniers, but they were small, whereas with larger panniers my heel will clip them. To get around this, unable to get a rack which extends back far enough, I attached the top section of an old rack onto my current rack:

So any saddled bag would be hanging 2 inches further back thanks to the extended rack space, and the angle means they are further clear of my feet. A bungee cord hooks over the straps of the pannier, then over/around the rear of the upper pannier to secure it.
Stronger strapped, large double panniers would be perfect - I find it a little too fiddly to deal with individual panniers

