OK, I am now in a position to give a more firm idea of the costs. I have more or less decided which Chinese tour company to use. This company was cheaper than most of the others. This worried me at first as I thought there may be a good (or bad!) reason why they are cheaper. However, I have been in contact with two (seperate) groups who used the same company for similar trips and my mind has been put at rest; I am confident this company is the right choice. The costs are:
Based on 6 people with 6 motorbikes from Torugart to Mohan for 35 days: total cost (including guide and vehicle rental but excluding hotels) is USD 16666.
That is approx. £1793 per person or £51 per person per day. If using our "own" 4WD (with 2 people) it is USD 11572.
That is £7545 divided by 7 vehicles is £1077 per person or about £30 per day. Although the 4WD would have 2 paying customers, as they would have to carry the guide, I have counted it as 1 person. If you see what I mean?!
Based on 8 people with 8 motorbikes (same route, duration, conditions): USD 18061.
This is approx. £1455 per person or £42 per person per day. With our "own" 4WD (with 2 people) it is USD 13225.
That is £8618 divided by 9 vehicles is £957 each or about £27 per day.
Sorry about the mixed currencies; the quotes were provided in USD$, but I think better in UK£.
Haven't found the 4WD to join yet; but the tour company has kindly offered to advertise for one on their website; so I still have hope.