Jesus mate, give him the pc for about £200 then sprint for the door! Once he gives you the cash. Dont even look back and if he knocks on phone the police.
On a more serious note, please forgive me, about before!
Maybe you should ask for £300 but once he is about to part, say i have changed my mind and you are a good friend and Just for you, you can have it for £200 smackers its worth blah blah with fitting and stuff, like a similar idea some i think mentioned before.