Well - assuming we're talking about the same kind of key.
My key is the black two button remote kind where the key folds away into the body.
As such, the metal part of the key is in two parts - the blade itself ( that looks like the Ebay kind ), and this seems pressed into a quarter round part which is embedded into the body ( it's this part that is pivotted so it can fold back in - will try to post a picture.
So - how do you remove the old blade part - and insert the new, cut, blade into the bit in the body.
Undo the screw under the button.
Remove the blade and holder.
Remove the very small roll-pin, this holds the blade in place.
As Mr Haines says "refitting is the reverse of removal"
BTW I don't know if you have a blade yet but buy carefully from eBay, I ordered the wrong blade. Mine had a number like A602 on it, this is the code for the shape of the retaining end piece they did a different one apparently that looks very similar but the retaining groove that the pins fits into is in a different place.
Finally, please post back here if you have success getting that key cut anywhere.
I've been banging my head for over a month now trying to find somewhere that can cut one for my 147.
Timpsons or everywhere else i've tried in a 30 mile radius don't have the bits small enough to do them.
the groove narrows to 2.3mm, most can only cut 3.0mm.
I currently have a nice keyfob c/w blank blade just sat here waiting to be cut