I wanted to report back now that I am safely in Angola for the next few months. The Houston Express was great - we were on the back up plane so there were plenty of economy seats. I had a window row (3 seats) to myself and it seemed like some business class passengers were instead in the middle row (4 seats) - I base this assumption on comments here & because I was able to see they had better movie/TV choices, their food was exactly the same.
We arrived an hour late & no-one else was there. I just followed the crowd, I think ending up in a diplomat line but no-one cared, I went where I was told. Immigration was easy, my baggage was there almost immediately and all in all I thought the airport was fine. Certainly better than the likes of Damascus or Kiev when I first went there 4+ yrs ago. Now watch how I've jinxed myself for the next flight
The domestic flight the next morning was a different story but all my luggage arrived on the same flight, the toilet was clean when I used it before we took off, and the sandwich was edible so even that wasn't so bad.
Thanks for all the tips - I admit I was nervous but in the end I've had far worse experiences. Now to get used to sub-Saharan Africa for a few years