Lubbie,
 
From 1999 foor VAT purposes to be a commercial vehicle a double cab pickup needs to have a payload weight ex works of at least 1000kg (and if it has a hood over the back pickup section at least 1045kg and say 1050kg for margin or error). SMMT members will usually get this information from the manufactorer at delivery so askthe dealer for more detail in specific circumstances.
 
From 2002 the Inland Revenue's treatment is the same as the PAYE treament in terms of the definition of a private car. With new VAN benefit rules coming in "private use" and agreements between employers and their employees banning private use may influence all this so care is needed and employers may be asked to demonstrate the position re private use (or the restriction of it) in one way or other. (see IR 480)
 
Hope this helps
 
Baz

Ahh did a search in HMRC website on 3500 kilograms and here's the link..
 
http://www.hmrc.gov.uk/vans/employer-van-facts.htm