It is possible to do it yourself, but you must register your intent to carry out DIY electrical work with your local authority, as both DIY and electrcal works outside in the garden come under part P of the building regulations.
Once you have paid the appropriate fee (normally around £200 - £300) and have been given the consent to proceed, you can alter the circuit and run an XLPE SWA cable to a PIR sensor to control the lights. All the SWA cable glands must be outdoor rated.
Once you have completed the work, you will need to contact your local authority again and get their inspector to come and inspect and test your work. If its all compliant with BS7671:2008 standards and tests out OK, you will be issued with an electrical and part P compliance certificate.
See the links below for more information on part P of the building regulations.
http://www.donttakethep.co.uk/
http://www.esc.org.uk/fileadmin/user_upload/documents/public/leaflets/PartP-Web-Oct2012-1.pdf