I took my phone, locked to T-mobile to a High Street phone shop, the guy said he could unlock it. Looks like he met his match with my J700i He was totalled defeated and gave me
a car charger for it, more out of embarrassment one would imagine.
Is there software I can download to unlock it.?

Software not Safe Unlock method Unlock code is best one for Unlock Samsung J700I model mobile from T-Mobile Network.You can get the Unlock Code In the site http://www.wickedunlock.com/samsung-j700i/rs11wp9/
For the Samsung J700I model mobile...In below You can get the Free Unlocking Instructions For that model mobile..

Samsung J700i Unlocking Instructions

Warning : Do not enter any wrong unlock codes on your Samsung J700i mobile phones. If you enter over a certain amount of wrong unlock codes on your Samsung J700i mobile phone, your phone may get hard locked and sometimes become unusable.

Method 1:

1 - Switch ON your Samsung J700i with a NOT ACCEPTED SIM Card.
2 - Samsung J700i will ask for "Password".
3 - Enter the Unlock Code.

OR (if the Samsung J700i shows "Insert Correct SIM Card")

Method 2:

1 - Switch ON your Samsung J700i with a NOT ACCEPTED SIM Card.
2 - Don't bother about what's being displayed on your Samsung J700i.
3 - Compose : #0111*CODE#.

NOT ACCEPTED: We mean, to use a sim card from a network other than the network your Samsung J700i is locked to.
CODE: Its the unique Unlock Code for your Samsung J700i.

If method 2 does not work, you can try method 2 again but this time without any SIM card on your Samsung J700i.