What a fantastic machine! More's the pity you won't keep this little pearl for yourself; the Velo Vipers are one of my favourite bikes and really are a treat to ride.
Velocettes are highly desirable bikes no matter what the model (possibly excepting the LE sidevalve). This bike in particular is one of the more sought-after varieties - Venoms, Thruxton-Venoms and, as in this case, the smaller Vipers are considered the best of the Velos by most enthusiasts. 1961 might be a bit early for most Velo-buyers tastes, but it will still command a premium, especially given the condition it's in.
Anyway originality doesn't matter much to Velocette types; almost every Venom has become a clubman or Thruxton regardless of what year it was made or what it left the factory as *curse you velo people!*
For prices I suggest you get a copy of Classic Bike Guide because this magazine has tonnes of ads in each edition and if theres going to be a 1961 Viper you can use as a price comparison for yours, it'll be in CBG. Please be very careful and take the time to do some research because there are plenty of people out there willing to fleece you for such a magic little bike, and it would be terrible for you to be short-changed for your father's legacy.
Sorry I can't be more help but I assume you're in England and the prices there vary considerably from here in Aus. At any rate good luck and I hope you find an honest buyer who'll take care of it and not just chop-and-change it like so many velo owners readily do!
Regards,
Blue
