So apparently I've just been chiselled on a trade

Your best bet is to post here with your proof... http://forums.steamrep.com/forums/report/

I doubt there is any way to get your key back I'm sad to say :(

Thats considered calling out and is against the rules ...
Remove any links to profiles ...
And actually just close the topic in general .

Leave him a Negative rep if you have his profile here , and thats it ...

Firstly, there is a 'no calling' out rule here so you will probably want to edit the name out of your post or risk suspension.

Secondly, this may help you find the guy here. If he has an account here.

http://www.steamgifts.com/discussion/DEwma/find-user-by-steam-id

Thirdly, be suspicious of anyone who adds you directly on Steam. +rep on their profile page is often fake and they can delete negative comments.

Fourthly, also check people on steamrep.com

(Edit; Now that you have added a link to his profile it is clear that he has already scammed a bunch of people here. Probably best to remove it though...)

Speaking of key/game trades, I'd like to point out that even the indiegala site, provides a 100% way of trading keys, without the risk of being scammed. Just wanted to mention that.... Did a few trades in the past this way.

A rule of thumb.
Guy has -4 rep here already.
He contacts you first on Steam without arranging the deal here.
This means that you will say he will go first. If he declines, then say thanks and a farewell. If they want the stuff, then they put the chips down first, especially with this background.
Also, report him on SteamRep if you still have a screenshot of the chat discussion. Once a report gets confirmed, these scammers get a very big warning on their Stem profiles for anyone using Enhanced Steam. (And if you trade, then you essentially must use Enhanced Steam anyway.)
Edit: Oh, and never believe Steam profile rep. The profile owner can delete any comment, so naturally no negative comments are there, ever.

As talgaby said. This guy already has -4 rep for scamming. He seems like a huge douche, as he gave negative feedback back to the scam-victims.

Never rely on the steam profile for +reps, it can be easily manipulated (joining +rep for +rep groups and deleting any comments calling him out as a scammer). It's like every few days someone gets scammed this way and creates a thread.
Do you have his steamgifts rep page?(probably not) Always research on what you are doing 1st, if you dive head 1st into anything new, there is an extremely high chance you will be scammed. If people are easily scammed, more people will want to scam as it is profitable, so I hope you learn this lesson and don't keep feeding the scammers.

Summary (if you don't read the passage): Never give in to greed, do thorough research before trading (make the extra effort especially if you are new, check the steamgifts profile and make sure it matches to his steam profile, check steamrep for any infractions).

Thank you all for the fast response, noobie mistake :S.