Renault Laguna Hatch 3.0 V6 Auto 2002 - 1430 1215 1500
Does that sound about right Luke??
If so take off about 150kg For you and the Dolly and you can tow something around 1380KG's total. Though the heavier you have your lag the better.
The first is the normal weight of the car, the 2nd the 85% reccomendation max, and the 3rd the manufacturers maximum.
Ideally you could go upto the 1500kg just by adding more weight to the lag, concrete blocks or sand bags in the boot as far forwards as possible can do this nicely as if you use a dolly nosewieght wont be a problem

and you'd still be under 3.5Tonne so you could in theory tow another lag home with it
