In reply to GarethSL:
1) Is your allen key a good quality one or a piece of junk you got in a cheap set or worse still with an IKEA flat pack? If so, go and get the best allen key your money can buy. Cheap allen keys roun off easily, are manyfactured to poor tolerances and don't fit the socket as well and will wreck the bolt.
2) WD40 is a water displacer primarily and a lubricator second. Whilst at tool store buy a proper penetration fluid - soak it with this and leave for a few hours
3) try again.
4) if you round the socket completely, get an appropriately sized torx socket and hammer it into the now buggered socket and try again.
5) if this doesn't work, get a srew removal tool a bit like this https://i5.walmartimages.com/asr/936c5cf1-376a-4f5e-853b-f81ec9914433_1.544...
You drill a hole in the bolt down it's centre. Then twist on of these in.
6) if this doesn't work, carefully drill out the core of the bolt and try to retap the holes.
7) if it really all goes horribly wrong, you may be able to helicoil an oversized drill hole.
8) stick it on ebay and hope for the best. Only joking, don't do that, that would be a dick move.