Just found this if it helps
How long does alcohol stay in the blood?
On average it takes about one hour for your body to process one unit of alcohol. This varies depending on your body size, sex and the amount of food in your digestive system. If your liver isn't functioning normally, the process takes longer. One unit of alcohol is roughly equivalent to half a pint of beer, a 25ml (pub) measure of spirit, or two-thirds of a small (125ml) glass of wine. This means that one pint takes your body about two hours to break down, and a large glass of wine (250ml) about three hours - longer if the ABV (alcohol by volume) content is higher than average. So if you have seven pints during a night out, it could take as long as 17 or 18 hours to leave your system.
It's very important to think about how long it takes your body to process alcohol before doing activities such as driving or operating machinery. Even if you feel fine the day after drinking, you could still be over the limit. The legal limit in the United Kingdom is 80 milligrams of alcohol for every 100 millilitres of blood in the body.
Taken from
http://www.nhsdirect.nhs.uk/articles/article.a spx?articleId=853