Do you know the commission number of the car?

Contact DVLA.
There's a form you can download fill in and return,
State you are a previous owner and would be interested in purchasing said car back.
Done it before cost's a Fiver IIRC.

Good Luck.

Currently SORN - details below:

The vehicle details for TKX 812L are:
Date of Liability 01 10 2012
Date of First Registration 22 05 1973
Year of Manufacture Not Available
Cylinder Capacity (cc) 1998cc
CO2 Emissions Not Available
Fuel Type PETROL
Export Marker N
Vehicle Status SORN Not Due
Vehicle Colour YELLOW
Vehicle Type Approval Not Available
Vehicle Excise Duty rate for vehicle
6 Months Rate £121.00
12 Months Rate £220.00