if you can read a cooking recipe, then you should be able to build your own rig.
the techno_priests would have you believe that it's far more difficult than it actually is, in order to preserve their perceived level of power.
the biggest hurdle is the minor research needed to ensure that you have no hardware conflicts (ie: between the mobo / processing chip / etc)
for £600, a self-built would buy you twice the computer of any premade system. plus, it may be possible to simply reuse some of the components that you already have (ie: RAM / monitor / etc etc etc).
here's some google-fu:
http://lifehacker.com/how-to-save-mo...n-pc-511195742