Hi, I am sorry I am not an expert but I have completed my Tax Return in the uk for 2011/2012. As I have an account in Turkey which pays interest and as I am a UK resident I had to fill in the full Self Assessment not the short form. If you go online I think the relavant pages are F2 and F3, you need to claim Foreign Tax Credit Relief, there is a list countries on the site with appropriate codes, Turkey is TUR. I believe I had to pay an addtional tax in this country of 5% as I had already paid 15% in Turkey. HM Revenue & Customs calculated the amount I had to pay for me, they must know how much tax is deducted at source in Turkey, I must admit I had to phone them and get them to help to ensure I did not pay too much tax, I also had to look back at the exchange rates as my interest was paid every three months. I do not know how the system will work if Turkey calculates different deductions of Tax depending on how long you leave your money in for.
I hope this helps!