Help - my wife's gone off sex

My wife and I have been married for 26 years – but for the past 12 months we have not made love. Although we row sometimes like any couple, we still get on quite well and have four super kids aged 22, 19, and twins aged 15, but the spark in our relationship has long gone. I’ve tried being romantic – candlit dinners, flowers and boxes of chocolates – but my wife just isn’t interested in physical intimacy...