Hello, I have a basic physics engine running that runs well with a low amount of bodies. However once there is a lot of forces and lots of bodies intercolliding, I get a lot of 'sinking'. I have read some of the source and what I get from it is that is solves all the constraints (which are resulting forces on an object ?) simultaneously. However most of the code looks really arcane to me, can anyone point me in the right direction?

Yes. Just use an established graphics library. If you're on C++, I suggest Bullet.

Otherwise, you'll have to crack your head open. The code you're referring to is the constraints solver and has been subject of intense research over the years. It is my understanding code has been shared between physics libraries a few times in the past so this is totally not trivial.