How do I add a socket to a fused alarm spur?
The easy option is to install a radial circuit straight off the consumer unit, due to changes to the wiring regulations this circuit will need to be protected by a circuit level RCB device.
As a alteration to the electrical installation it would also need to be undertaken by a suitably qualified person. and  a Minor works certificate issued.

Failure to have a certificate could result in default on your home insurance.

Look for a NICEIC approved electrician who should be able to complete the works within a hour or two.