cheers everyone , especially jim , for your help , gonna start now , I'm like a kid in a toy shop at the moment , no doubt ill be in touch soon

am I right in thinking to get my -z value manually  I should measure the distance between the tip of the bit to the cutting surface and add this into my g code  immediately AFTER G0z5.000 .
for example G0z-10.000 ?

You could edit your Gcode manually but ideally the depth of cut should be set in your CAD/CAM software (ArtCam ?) before you generate the Gcode.

Within the CAD/CAM software it is usual to set Z0 as the top surface of the work then the depth of cut will be a negative value (for example Z-5.0 if you want to cut 5 units deep). Then, before machining the work the cutter is lowered to just touch the surface of the work and the Mach3 Z axis DRO set to Zero - the cutter is then raised above the work surface and the job is started.

Perhaps do a search for Auto Tool Zero - that should explain how to set Z zero and the tool to the surface of the work (If it's any help to you I have described the way I set tool position here http://www.cooperman.talktalk.net/files/17.htm )

Tweakie.