I always fill mine up with small amounts. More fuel = more weight :p Never had the need to vent but I don't often drive long distances.
 
how do i vent the tank it just wont take more than 36 liters

if you search venting you will find loads on it, basically look down the filler neck and the is a little black lever on the right push this down while filling and the air in the tank is replaced by fuel. if you have a petrol this is not reccomended
 
how do i vent the tank it just wont take more than 36 liters

As said above - take the fuel cap off and you will see a little black toggle on the right hand side. When the tank is filled normaly, hold this toggle down and slowley (about 1/2 speed) fill the tank. I can usualy get 12 - 13 litres extra in doing it this way. Note that the auto cutoff wont work, so you will need to watch the level!
 
Filled up my new car today (PD130 with 45 litre tank) and got 40 litres in without venting... realised afterwards it was VPower diesel :doh:
 
mate if you go on the auto trader and find a car the same spec as yours, theres a section where it tells you the fuel tank capacity :)