I have the same van and it started doing the same as yours last week. Have had numerous problems with relays and the connections to them so hoping its the pre-heater relay. Otherwise the pre-heaters may be tired... Either way it will always eventually start after about 30 seconds on the starter motor...(assuming you have a good battery)
Also, my starter motor solenoid doesnt work when wet (ie after driving through water)...Prob not related to your prob though... Good luck-anything you find may also be helpful to me... rob underscore hargrave at yahoo dot co dot uk (i dont check this forum)...
Rob
October 2007