Hi Ian
Personally I would take some Turkish lira with me - at least enough to start the holiday with, for first night drinks and so on.
As you should say, it shouldn't be a problem using the ATMs over there. Remember to let your bank / card issuer know your plans in case they block the card.
As for excursions, you should have received a letter from us recently outlining any optional excursions, how much they cost and how you can pay. If this letter says you can use sterling, then that will be fine.
Have a great trip - Istanbul is an amazing city!
Kind regards,
Olly, your editor