Steve,
from my experience whilst the banks often don't charge a fee, the rate they give you is much lower than the local rate in Calis.
Personally, I draw sterling out of the HSBC cash machine and change into TL locally. For example, Natwest charge £2 for an overseas withdrawal when you do this so thats the only penalty. Only makes sense if you get a decent amount out as I think the £2 is the same for a £10 or £500 withdrawal!
It all depends what the fees and rates are for your particular bank account.
Ian