Welcome to the forum from Louisiana, glad you found us.
have read you should only introduce pairs?
We all have different opinions on this. I'm not convinced that's very important if they are all adults. These are old enough to make their way into the pecking order. This may be violent but it's usually over pretty quickly. With immature chickens, which you have, I think it is very important. One theory is that if you add more than one the others will spread the abuse out. That's not what I see. Some are more likely to be picked on than others, regardless of how many are being integrated.
Where I think it is important with juveniles is that with my set-up the juveniles tend to avoid the adults until the juveniles mature enough to force their way into the pecking order. With my pullets that's usually about the time they start to lay. Until then if they invade the personal space of a more mature bird they are at risk of getting pecked. It usually doesn't take long for them to learn to avoid the older birds.
Chickens are social animals. They really like to be with other chickens. If you are trying to integrate a single chick they may really want to be with the others but they may get pecked or worse if they try that. If they have a buddy they can enjoy each other's company while avoiding the older birds. You can get a similar situation of the older bird is the single bird. It may initiate the contact and then peck because they are too close. It's not that chickens are dumb, their logic just doesn't always work the way ours does.
We don’t really have the space for 4 chickens but one more would be ideal, is this possible and what would be the best way to do so?
To me this is a huge red flag. On what do you base that you have enough room for three but not enough for four? Some numbers that you read on this forum? I've seen numbers anywhere from 1 to 15 square feet per chicken in the coop with 4 square feet being most common. Sometimes there is a number given for in the run, often not. 1- square feet is a common one. So which number do you choose? If it is four square feet in the coop that is for chickens that are already fully integrated. Often it is more than the absolute minimum you need, sometimes it is not quite enough. We all have different conditions and circumstances so we can get different results. If you have 50 chickens the 4 and 10 numbers come up to a lot of room. If you have 3 chickens it doesn't.
When you are integrating you need more room. One way chickens have learned to live together in a flock is that when there is conflict the loser runs away. The winner may chase a bit but usually they back off if the other runs away. That's one way they determine the pecking order. If the loser doesn't run away the winner doesn't realize the loser has given up. They keep attacking, usually trying to peck the head because that is where they can do the most damage. That's a good way to wind up with a dead chicken. It is extremely important that they have enough room to run away and get away. Say your coop is 3' x 4', which according to some popular numbers is perfect for three chickens. Say your run is 5'x 6', which according to some popular numbers is ideal for three chickens. How far can they run to get away? How can juveniles avoid entering the personal space of the more mature birds?
If you have a lot or room you can often just turn them loose together and they will work it out. That's a common tactic when they free range or you have really big coops and runs, they have lots of space then. But most of us don't have that. There are techniques to help us integrate even if we don't have much room. If you can give us the size in feet or meters of your coop and of your run and maybe show photos of what that room looks like, inside the coop and outside, we may be able to offer specific suggestions that can help you.
The hen we already have has such a lovely nature and will come jump on your leg for some cuddles, will adding another hen change her behaviour at all?
No matter how sweet and cuddly a chicken is to you, they can still be a brute toward another chicken. That is no indication of how they will accept another chicken. When you add a chicken to the flock you can change flock dynamics. But that is unlikely to change how that hen responds to you.
The cockerel has only just found his voice so we presume he has finally hit maturity, would that be a problem too?
Maybe, maybe not. A good mature rooster will likely accept a new hen into his flock and may keep peace within the flock. A good mature rooster often makes integrating a mature hen easier. But not all mature roosters are good. And your chickens are not mature. An immature cockerel can be a hormone driven brute. I agree, you don't know until you try it.
Good luck and once again
their behaviors should stay the same, they might spar a little at first but once they settle in they should be fine, unless the pair totally reject her. As long as they have a little space to run around feom each other they should be fine 
cheers
