Speaking of the devil. When I put my Skew socks on today, I thought about asking you how you changed the instep. I made mine according to pattern, since I have a low instep, but they are quite tight at the back of the heel.
Mine always remember me of last Forum since I made an international picture of them in Reia's room. Danish yarn, pattern from a Canadian magazine, English company and food from at least three other countries between our feet in the Italian room of someone from Alaska.
Harma, I just stretched out the increases over a longer bit of the foot - instead of working to the 7 or so cm in front of the heel before starting the increases, I staggered them out from where my instep starts and just worked in more plain rows between all of them. That, however, won't give you more room at the heel - I guess that for this you would need to change the heel to graft more stitches together, or to make it a bit longer.
You can also find instructions on how to add a short row or two to give more heel room on the Skew inventor's blog: http://yarndiva.blogspot.com/2010/02/easing-into-it.html
Have fun!