We are British and came to Turkey in August. We are coming again in October. Is our entry visa still valid, or do we have to buy another one on entry?   

Your visa is valid for 90 days, so you have to count 90 days from when the visa was received to learn its exact expiry date. If this period does not cover your holiday you will need to obtain another Turkish visa at the official point of entry (e.g. International airport, seaport or land border).