Ian, have never paid extra tax when changing to different currencies, the tax has already been taken from the monthly interest account, you get the rate at time of exchange.
BBee, cant really answer your question because if you wait for best rate there is no telling if that rate will go up or down, and what is happening to your money while waiting.
Another thing to remember while waiting is if lira weakens against the pound it could add as much as 25% or more spending power to the 5% you get from a stirling account.
Its a very big subject and lots of different ways to manage your cash, what has worked very well for me over the past five years might not work as well for someone in different circumstances.
I would hate to give advice that might cause anyone to make a loss.
Off now to speak to KKOBs Nigerian friend:D