From the FCO website
Entry Requirements - Passport validity & space for visa stamp
You must hold a valid passport to enter Turkey. Your passport must be valid for a minimum period of six months from the date of entry into Turkey and have at least three months validity from the date you are exiting Turkey.
You should ensure that your British passport has a blank page for a visa stamp. If it does not, the Turkish Authorities will issue a one-off “Entry-Exit form” and put your entry-exit stamp on there. You should carry this form with you in Turkey and present it to the border officers, when you leave Turkey.
After 31 December 2012 all foreign (including British) passport holders will be refused entry to Turkey if their passport does not have a blank page for the entry visa and entry/exit stamps.