My hubby and I are trying for our first child, we have only been trying for the past 3 months (however we have not been on any contraception for the past 4 years, he allways used the withdrawal method)

For the past few months I have been monitoring my cycle and have noticed that although my periods are regular every month they don't allways come every 28 days or every 26 days etc, one month is different to the next. One month might be 26 days the next might be 29 days and so on. So each month I don't know the exact date the I'm due on. But doesn't this make things a little more difficult as if my cycle is different every month won't that be harder for me to determin when I'm ovulating ?

Regardless of how long your cycle is, your ovulation can vary on it's own as well. Even if your cycle was 28 days every month, you could ovulate at different times.

Here is a site that can help you find your window of ovulation... it is more accurate in determining more than just one day that you could ovulate... thus gives you a better chance of not missing ovulation.
American Pregnancy Association : Getting Pregnant: Ovulation Calendar (http://www.americanpregnancy.org/gettingpregnant/ovulationcalendar.html?menstrual=07%2F28%2F2007&cycle=)