I have been researching the Trotter's van. There seems to have been more than one registration used on the Reliant Regal in different episodes of OFAH. The registration DHV 938 D seems to be the most prevalent. I have found the following:
http://www.trotters-independent-traders.co.uk/ episodes/a_touch_of_glass.htm
On the above site, Danny Zane Clements comments that in 'A Touch of Glass', the 3 wheeled van's registration is APL 911 H whereas in 'Modern Man' the van's reg is DHV 938 D
http://www.thevervoid.com/media/onlyfools_20.h tm
On the above site, in the episode 'Yesterday Never Comes', the registration is DHV 938 D