No not part p, dont you have to test the 

Whole house if you add a spur?

 

You just have to be a competent person to fit a fused supr providing it not in a special zone such as a kitchen of bathroom.

You need a test meter to check for earth fault loop, rcd test, insulation test etc, and have to issue a minor works cert for the spur.

Tavcom in Southampton do a day course for it.

 

don't really want to have to test the whole house just to fit a spur,

You don't need to test the full house...

If you adding a new circuit to the consumer unit then you would need to carry out a full installation cert for that one circuit you are adding.

If you are adding a spur to say a ring main then you can carry out a minor works cert.