What the PWM does is effectively couple the solar panel directly across the battery so the battery drags the voltage down. The graph below shows a typical curve for current against voltage.To answer my own question. Please correct me if I'm wrong.
Now it's all hooked up, tye voltage is down to 12.6 in the roof, and 12.6 at the input to the controller, my understanding is now that the open circuit voltage (Voc) is that voltage when it's not connected. The maximum power voltage (Vmp) will be the biggest it can give to the controller and the controller sets the voltage at any point in time to just above that of the batteries, thus creating a charge. The MPPT, would do the same just with a few more amps as it does something a pwm doesn't to up the amps as it drops the volts.
Anyway, we have a working 100w panel and PWM controller again, which I will rewire when I move it to make space for the new panel. Thanks again for all your input, I wouldn't have had a clue without you all. You're stars!
Now, I could get the laptop out and get on with my tax return (which is what I was going to do this morning, but needed power), or could have a beer, hmmmmm, decisions!
Power is volts times amps and for your panel maximum power is at 17.7 volts when it is generating 5.7 amps. An MPPT controller will take that 100W of power and generate say 14.4 volts and 7 amps from it. A PWM controller can't do that and the current it can generate could be read from the graph above - if it had numbers on it of course. But at 12.6 volts it might be generating about 6 amps which is about 75W or 25% less power than the MPPT can generate.