Just my 2 cents. The trouble with trying to -thermaly shock- it either with 'freeze & release' or the opposite 'a hot blue-spanner' is that the body of the fuel pump that it screws into will act as a bloody big heat/cold sink and thus have a -marked- dampening effect upon just what it is that you're trying to achieve. That's not to say that you couldn't give it a try of course but, the other thing to always remember is that if you get far -too brutal- with a 5mm screw/bolt, then you are very likely to just shear it off, and that possibility, in this case, dont bear thinking about.
Just personally speaking, I think I like coughy's -voice of experience- suggestion best so far ie,
"by coughy on Fri Jan 09, 2015 4:53 pm
get a socket allen key that is of different size (ie i think it is metric 5mm so get the imperial 13/64) and smack it in there also hit the head of the bolt as hard as you can with the hammer then try and tighten it up first then hit it again then try to undo it. As the imperial 13/64 is 5.1594mm compared to 5mm allen key little bit bigger so it will bite in but the trick is to smack it with the hammer and i mean smack it i do this kind of stuff at work every day removing bolts like this. are you using the allen keys or the socket head keys. make sure there brand new so the corners are still on the hex."
Might even be worth PM-ing him for 'detailed' instructions...?