If its off Withams it should have the cast certificate which normally is needed to register the beast. It can be done another way through the MVT vehicle verification officer local to you for a fee. Withams for some reason seem not to want to pass this on.
The plates for vehicle weights, axle loadings and chassis / vin number is on the drivers side step area. Its worth taking a pictures of the various plates to forward to the DVLA / Vosa. Do you plan on converting into a camper or r you planning on leaving it as it is? Is the comms body empty or full of Clansman type of equipment? Depending on what you want to do it will either have to be MOT'd and plated at a VOSA test station or if converted into a camper and then MOT'd as a class 7 motor home.