According to this www.pcspecialist.co.uk/faqs/ you won't void the warranty simply by opening the case and installing your own components. Of course, it's a different matter if you break something but there's no real danger of that if you're careful.From what i understood, there are 4 slots for RAM, two that are on the opposite side hidden under the keyboard and then two under the access panel on the base of the machine....
I am really surprised that PCS would use the easy-access ones rather than installing it in the "hidden" slots during building.
From what i know, the only way to access the slots under the keyboard is to physically open the casing...this often marks the case and isnt something i would recommend unless you know exactly what you are doing. Contact PCS and see if they will deal with it for you or take it to a nearby computer shop and have them do it, last thing you want is to cause damage unwittingly or find you break your warranty for opening it up like this... (i dont know if it would, but it is quite common to have clauses like that in a warranty as you can do all sorts of damage)