Did you have any personal or financial information on your computer's hard drive when you took it in??? He could have not only stolen parts, but decided to deal in a form of Identity theft. :( :( :( You wouldn't necessarily know what he did because the info remained intact on your computer, but he could have sold it on to others. :( :(
Identity theft in the US is a growing problem --- people's Social Security numbers are used in credit card applications and cell phone purchases. Thousands of dollars are racked up in an unknowing individual's name. I don't know what is used for ID verification in the UK, but I would be cautious.
At the very least, you might consider calling the police to let them know about this incident. You could find out if this guy has a 'rap' sheet or it might stop him conning someone else. :( :( :(
I'm sorry this happened to you.
Enjoy your new computer :)
toodles