Strangely enough - if you are British, American, Australian, Canadian, Irish and another couple I can't remember, then you will need a visa. If you're Japanese, German, French, a New Zealander or a lot of others then you won't.
Contact the Turkish Embassy in London for a visa or alternatively get it at Passport Control when you arrive in Turkey.
On arrival in Istanbul, join the queue and get a visa at the visa desk before you go to passport control. If you have a British passport you will need �10 in sterling (not Scottish notes) and I think they take euros now. It's no problem, just a formality. They will stick a stamp in your passport.
you dont need a visa, but when you fly into turkey you have to pay ten pounds for a sticker in your passport. make sure you have a ten pound note as they will not give change and are not keen on loose change.