gavin, I had a 2002 petrol and now have 2005 diesel. I do not know why the 2002 has such a low nose weight for vehicle type but I know why it was upgraded for the 2005 variant.
It was as the result of feedback from owners of the 2002 variant saying that they would not be replacing them with another because of a) the noseweight limit and b) because there was no diesel variant. They listened and in my opinion the 2005 diesel is a very good all round car.
I had the same problem as yourself having a 5 berth with MPTLM of 1350kg. It was a struggle to keep it near 60kg and if I had packed the van as I wished it would have been about 85kg.