Do you know if the ST70 was previously road registered?
If it was then:
Either the V5 has been lost by the previous owner, in which case the DVLA can help.
The bike was stolen, in which case the DVLA can help!
If the ST70 was not previously road registered you need to write to Honda UK with the VIN. For a fee they will provide a certificate giving the date of manufacture. This is needed by the DVLA to road register the bike with an age related number plate.