Whats on the V5 at the moment do you still have part of the V5?
The vin or hull number suggests that you’re Ferret was built in 1965 (no V112G MOT exemption for you)
Unladen 3683 kg, Laden 4394 kg
Info taken from page 18 of the 1969 User Manual pic attached.
Go to the council weigh bridge pay £10 pounds or so for the certificate of them, the DVLA will except that as proof of weight.
Its odd that they are now asking for the weight and VIN / Hull number given that you are the fifth owner. The build date 1965 might be the reason that they are asking for this info