They weren't when i fitted my Cav leather as the split is different, the 2 parts were different sizes. Basically what i did for the seat bases (as i'd just had enough and wanted them in) was weld the 2 parts of the Cav base together using a couple of strips as i wasnt fussed about having to lift the whole rear bench as one and it made things easier in the long run as due to the bases being a different shape on the underside they wouldn't have been as stable.
drilled out the welds for the brackets, measured up then welded 2 of the Astra base brackets to the Cav bases so it was only mounted using the 2 outer-most points, but it was more than solid enough.
The Cav bases didn't sit quite right on the Astra pan originally, but once all welded together you wouldn't have known unless you lifted the rear seats up.
Can't actually find the pics i took at the time, but Speedysam has my Astra now so he might be able to get pics if you PM him