The passport agency website recommend six months but when we booked our holiday a few weeks ago with Thomsons, we were advised (and it was confirmed on the paperwork) that we should have
at least 3 months left on our passport after our return date (we are traveling to Majorca). I think it does vary from TO to TO/country to country but I think its worth renewing so you know that you will have no problems.
Our passports ran out about 5 months and 4 days after our return from holiday last year which was a real pain considering how close to 6 months it was. Even so, we renewed before we traveled and the extra few months were added on to our new passports. In effect, we have a passport that lasts for 10 years and 7 months.