Thankyou for all of your replies!
DH and I had a chat and have decided to wait it out. I don't think it will be too likely that I will fall pg, but if I do, then it is obviously meant to be. I don't want to mess with my milk supply by taking any pills as my supply is hugely affected by my hormones (DS2 gets terribly fussy when AF is around, and DD and DS1 both fully weaned themselves within a few weeks when I fell pg again) and to take the pill, even just momentarily, I am scared would make DS2 possibly wean or leave him hungry since he's not too into solids yet which is totally not fair to him.
While DH and I aren't planning on having another child, nor necessarily want another (by choice!), if #4 is on the horizon, then he/she will come no matter what! So be it. DH would never put any kind of blame on me since he was half of the equation! Whilst I often joke about him leaving me or being angry if I was to fall pg again, he knows full well that he is part of the plan too and I know he seriously wouldn't take it out on me or any potential child. We would never regret any child that is brought into our lives. Although I think if I were to fall pg under the circumstances, then it would be one tough and determined little cookie!
I must just say though, we are usually pretty careful! Not that we get to get busy so often these days, lol! But this is the first time since having DS2 that we have been so...'irresponsible' isn't quite the right word, but I'm sure you get the picture! I would definately not reccommend using withdrawal as contraception unless you don't mind having a child/more chidren. DS1 was concieved using it and though we didn't intend on having another child so soon after having DD, we did plan on more chindren, so weren't being REALLY careful. Though I was vaguely watching my cycle and using the withdrawal 'method'.