I don't think you can SORN unless you are the registered keeper.

From the DVLA site:

How to make or renew a SORN:
If you have become the registered keeper, you can make or renew a SORN in the following ways.
Online
• If you have a V11, an ‘Application for a tax disc or SORN (Statutory Off Road Notification) for a heavy goods vehicle’ (V85/1), or a registration certificate (V5C) in your name, you can do it on our website
at www.direct.gov.uk/taxdisc
By phone
• If you have a V11, V85/1 or V5C in your name,
you can phone 0300 123 4321
In person or by post
• You can take your V11 to a Post Office® branch that issues tax discs, or take the V85/1 to a DVLA local office, from the 5th day of the month the tax disc
or SORN runs out in.
• If you are applying by post, you should return the V11 to the Post Office® branch shown on the form or send the V85/1 to a DVLA local office. Please do not send the form to us at DVLA, Swansea.
• You can use a ‘Statutory Off Road Notification (SORN)’ form (V890) which you should send to
us at DVLA, Swansea, SA99 1AR.

jamie745 is correct, if the car is down as "in the trade" with the DVLA then it does not require a SORN.

A trader only needs to SORN a car if they register it in their own name.