Ian you are better off going to a local market.There is normally at least one stall that only deals with mobile phones and unlocking
them.I dont know any shops as you say that will unlock a mobile
phone and if they do they will charge a lot.The web sites are too
much hassle anyway as you have to send your phone to them.Markets
are your best bet.I still think you should find out for sure if
you can still take a mobile to Turkey and insert one of their sim
cards and it will work.Preferably before you go