Also, whilst I will obviously take a back up, am I right in saying that in theory the data should be left untouched by an unlock? Is there anything else I should be careful of?
FYI they are on TMobile, but contracts have terminated - so not only will T mobile not unlock them but also it takes 28 days and I don't want to wait that long really.
(Should have had them done before I came - most high streets in London has stalls etc which will unlock phones for $18. I gather it is more likely to be $50 here!)