Selling Car Documents

[sorin1987]

This is the first time I am selling a car since I moved to UK.
I read online that beside the V5C documents you have to send to DVLA (or let them know online that you no longer own the car) you can have a Seller Contract where both parties sign it. It can be downloaded online from a lot of sites...
Is this contract absolutely necessary? I am selling the car to some good friends and there are no trust issues between us. The online guides I read don't actually mention if this step is mandatory or just an option to reasure each party.

Thanks

[sorin1987]

Does this contract have to be signed in the presence of an official witness? Like a lawyer or notary? Or just my wife being there is enough?