Alan,
Whatcars database does not include the 534 but matching their data for a 1400 kg lunar and a 406 "break" 2.0 HDI suggests an 88% match and an acceptable tow.
Several owners report favourably on their 406's performance towing various weights up to and including 1400 kg.
As for the 5th gear aspect you would really need to talk to other Pug 406 owners - speaking fom experience of towing with various cars, 5th gear is usually only usable on the flat above 50 mph and a change down with plenty of anticipation is necessary to maintain momentum.
For example on my current tug, a 3.5 litre auto Shogun, towing a mere 1500kg; I find that a flick into 4th is benefical to keep the speed up on motorway inclines. I have also found this to be true with diesels although generally they do hang on to the revs longer and down changes are less needed.
Hope this helps but if you are sure the loadin match is about right then maybe ther is a problem with the van like binding brakes or you could get the car's power checked on a rolling road.
Just a thought, is the cars turbo working OK? Diesels go very limpwristed without them!