You can locate the seating chart for your reserved trains here:

www.lefrecce.it

You will need to locate each train's seating chart separately. Charts show only by individual railcar, and (as far as I know) you can't force the system to show you any particular railcar chart. It bring up a certain railcar according to its programmed logic. Let us know if the chart shows the seats as being unavailable (assuming that it shows the specific railcar where your reserved seats are located).

In any case, don't worry: you should be able to trade seats if necessary between Florence and Rome (nonstop on the AV train) and between Rome and Civitavecchia (nonstop on the ES-City train).

Thanks for the fast response. I am obviously having a bad day as I can't see any links to seat plans from this website. Can you explain where they are located?

Next time use the Le Frecce link to book as you can actually click on the seats you want. Here is the link in English.
http://www.fsitaliane.it/cms/v/index.jsp?vgnextoid=1d85c6b9455c2210VgnVCM1000003f16f90aRCRD

You have to be sure to click select the seat (scroll down after you select your train) and then continue without registration(if you just want to see diagrams). You will be able to see seat charts on the next screen page. You might not get to see your particular car (each can be a bit different) if it's already sold out as Trenitalia will only show cars that have availability.

Thanks. I have found them.

Also discovered that I hadn't noticed that I had inadvertently switched stations. Unfortunate as we are staying within walking distance of the SM Novella station and could have got a train from there. We now go from Campo Di Marte One lives and learns. This is a much better website!!!

Both links above are for Trenitalia's website, but only for the faster trains. It will not show the slower R trains. I always check the regular Trenitalia link and the Le Frecce link as I've found fares to be inconsistently loaded between the two. If you use the link for faster trains only, you can see multiple trains and mini fares on one screed. You also have to use the Italian spelling for the stations (even on the English) page or you won't get results.

You have fallen into the trap of inadvertently booking one of the rare AV trains from Firenze Campo di Marte to Roma Termini. You're not the first, and will not be the last, to fall into this trap.

If I were designing the Trenitalia website, I would flash a warning legend: "You are departing from Firenze Campo di Marte" before somebody proceeds to book the ticket. But the site it designed primarily for locals, most of whom know about the different train stations in Florence (and in Rome).

Well it is nice to know that I am not the first to be so stupid.