Many thanks to everyone who has replied. You have all made me feel very welcome. The advice has been very constructive indeed. I now think that the 16:05 to ORD is a definite non-starter to connect to MSY that day. I think I always knew this would be the case but did wonder if I was missing something but I am not as you all highlighted the things that were worrying me as red flags. The AA flight to MSY is the last flight of the day so miss that and we have had it. My sister will be 65 when we fly, and neither of us is as quick as we were, but I don't think the assistance thing will buy us very much/enough time. It wouldn't need much of a delay for the connection to fail be that a flight, the weather, immigration, baggage claim or customs. Too many variables outside of my control.

I think I need to look at a Plan B. Or Plan C. We will consider flying via DFW staying at LHR the night before or Arrive ORD stay overnight then head for MSY next day. I wouldn't mind a day or so in Chicago but we are looking at either November or January here so that puts me off that idea. I have flown into Miami 2 times both awful so I don't think we want Miami unless it is the only option that works!

Thanks again you have all been great and very helpful in my decision making.