Sorry for being so "dumb" but could you tell me if the circuit will definately give +12V to one side of the solenoid and 0V (ground) to the other please?
once the pic chip turns that pin high which I presume is +5v or thereabouts.

I've tried to get my head around what you said but I'm getting thrown through by

If Vgs tries to exceed -12v, the Zener breaks down and conducts current to keep the Vgs around -12v.

Promise this will be the last "dumb" question I ask for quite a while

 

Yes, when the MOSFET is turned on by the gate being pulled low (thus Vgs becomes -12v) the drain of the P-channel MOSFET will source current to your relay coil. The other end of the coil is connected to ground, so your electrical systems' voltage will be applied across it.

The 1N4004 diode is to take care of the reverse-EMF pulse that occurs when the MOSFET is turned off. The relays' coil acts as an inductor, and tries to keep the current flowing as the magnetic field around it collapses.