Flameport; your probably right that it won't save anything to run the cable myself and I probably will get the sparky to do it. However, I do think your a little overzealous in your application of part p.
I being the person responsible for the Design, Construction, Inspection & Testing of the electrical installation (as indicated by my signature below), particulars of which are described above, having exercised reasonable skill and care when carrying out the Design, Construction, Inspection & Testing, hereby CERTIFY that the said work for which I have been responsible is to the best of my knowledge and belief in accordance with BS 7671:2008, amended to .............(date) except for the departures, if any, detailed as follows.