Thanks very much for all your replies. I'm starting to get a better handle on how much time we need. Unfortunately when I started researching I used tour company itineraries as a rough guide of how much time to spend in each place - most trips doing trans-sib seem to spend 2 days max in Mongolia. Obviously, on closer inspection, it seems worth spending a bit more time. On the other hand, we can't spend 3 weeks in Mongolia as we have a finite amount of time to travel.
I've now discovered the public bus to Terelj NP, so that will be a day trip, and we will do UB ourselves in one day. I guess if we allow 6 days for a tour, that might just be enough. However, we will probably have booked our trains in advance so will have a definite departure date. We therefore have to book a tour quite quickly after arrival in UB - are next day departures generally possible for tours?
I also get the impression with these jeep tours that if you have a full-jeeps worth of people, you can basically dictate the tour and have everything tailor made. Is this the case?
Finally, one more question - is it possible to change departure date of our train tickets to Beijing once we are there? Are there extra costs for changing the ticket if so? We don't speak any Mongolian but do speak basic Russian. We are going in September so are aware that they may be difficulty in getting tickets are short notice.
Thanks very much for your help!