I currently weight 154.3 (70kg) and about 2 months ago started weight training again after about 4 months off. I was wondering how I can add muscle without gaining the extra fat that seems to come along with it. I usually struggle gaining muscle and eat more to compensate for this but I don't want to wither away again by just focusing on cardio.
In the past I have weighed 183 (83kg), but had too much fat on my stomach and don't won't this again. I do about 3-4 sessions of weights a week, along with 1-2 sessions of cardio training lasting 1hr each. Any ideas?

Diet is what you have to get under control when bulking. You should expect to gain *some* fat when you go for muscle gain, but you can minimize it if you watch what you're eating. Start at around 500 calories above masintenance. If you see no muscle gain, increase the calories by a couple hundred a week until you do. If you start to see excessive fat accumulating, drop calories by a hundred or so until that stops. Keeping your diet clean is also going to help. Minimize the junk food and keep a lot of whole grains, fruits and veggies, lean proteins, and healthy fats in the diet. It really does make a difference.

Although you don't want to do cardio, it will help... Don't do a lot, but about 20min of intense cardio 2 days a week will help reduce (only a little though) the normal increase in body fat % which goes hand in hand with mass building. If you can go swimming, do this instead of other cardio as it helps with resting your joints between heavy lifting days over other forms of impact cardio.
Unforunately, what you may have to do is just accept that you're going to gain a bit of body fat while you're bulking up, and then strip down your diet once you're a few pounds above your ideal weight to tone and reduce the body fat % while maintaining the lean muscle.

From what I understand a minimum of 45mins is needed for CV exercise to have any affect. Is this correct? 20mins doesn't seem enough.

The thing is, when you are bulking, you need to have a surplus of calories in order for your body to be able to create new muscle tissue. If you start doing a lot of cardio, you're burning off fat AND muscle (although usually a larget proportion of fat). This ends up being counter productive. 20-30 minutes is a fair amount and is not totally ineffective, but you WONT have to do much cardio IF you don't allow the calories to get out of hand in the first place. You don't have to burn off fat that you don't gain. You'll probably want to do some kind of a cutting diet after your bulking phase anyway, but just watching what you eat will make it so much easier. Cardio alone cannot do it while you're muscle building if you're just eating out of control.