Is there a free way or calculator i can use to unlock a sony ericcson k770i please?

No calculator but i can help you unlock it for free .....

have a look here http://forums.se-nse.net/topic/20668-who-wants-to-unlock-k800-db2020-cid52-for-free-with-supplied-dcu-60-cable/

that shows a k800 but your k750 would work the same way except you would not have to bypass security.

also are you aware that a k750i can be flashed to a w800i?

tutorial on that is here: http://forums.se-nse.net/topic/3996-k750i-to-w800i-via-seus-and-dcu60-for-free/

personally if it was me i would flash the phone to w800i then unlock it, in fact i have down that as i used to have a k750i and so did a mate of mine which i also did same thing