I am looking at buying a LWB van are Fiat Ducato vans any good, what are the biggest problems
Please help me decided Fiat Ducato, Ford Transit, Merc Sprinter or
VW Crafter

I'm delighted with mine. (Citroen Relay, same as Ducato H2 L4) The wide body and low floor makes it great for a camper van.
Low floor and ground clearance not so good for a builder though if you are going off road.

Thanks no off road work,
Wouldn't mind converting to a camper though
When I buy it I will put roof air and awning straight onto it

I am a bit worried about gear box rear shudder and the bad service from the Australian importer

No idea about Australian importer, sorry.
Mine was manufactured in May 2008. Reverse gear is a little higher than would be ideal, but I doubt if I would have even noticed that if I hadn't read about it on the web. I have spoken to about 20 X2/50 (Ducato/Boxer/Relay)commercial van drivers, not one had even heard of the judder, and every one was happy with their van.
It seems to affect motorhome drivers more than van drivers. In any case those manufactured after late 2009 apparently have a lower ratio reverse gear.