Hi, i need to chase a cavity wall for a plug socket. Where do i start? can i first drill holes in the marked place for the plug or must i first use the grinder?

ITs not necessary to use a grinder. The box for the socket can be marked out on the wall along with the cable chase. A series of holes can then be drilled in a line along/inbetween the markings, before chasing starts. This will make it easier to chases and reduce the risk of the edge wandering and a large chunk of unwanted material being removed. Be carefull as its easy to break through into the cavity. You may need to cut a small piece of wood to create a "noggin" to support the socket back box. Remember tha under BS7671:2008 all new sockets that are added to a domestic installation now need to be RCD protected so if your existing fusebox does not have a BS61008 RCD or BSEN61009 RCBO protecting the sockets already, you will need to supply the new socket via an RCD switch fused spur or use an RCD protected socket. Remember to register the work with uour local builing control department before you start and pay their inspection fee (around £100 to £300) so that the work is fully signed off under part P of the building regulations. As the other poster has highlighted, its probably far cheaper and quicker to get an electrician in, i'd say that the whole job would be less than £50.

get in a part P registered electrician, if you need to ask such a simple question then I doubt its wise for you to be tampering with the electrics.