" evidence of the vehicles' date of manufacture (latter must come from manufacturer). "
Check your tractor serial numbers, you may find a date code, to quote to the manufacturer, or can you obtain the importation date ? Did your dealers cert include the tractors vin / serial no ? is this the importing dealer ? if so they should have the importation document and certificate of newness after all they now have the Cof C.
What type of registration are you applying for ? bearing in mind this vehicle is 3 years old , are you attempting to register as new , or applying for an age related registration or a "Q" plate, and finally a not dumb question ,what are you trying to register it as ?
Form V55/5, just pop along to a new car/tractor dealer , they will have some ,ask nicely and they may let you have one. Make sure the form is the new one V55/5 8/16