And had a play with the fan switch off my donor manifold. Turns out the 1.8 cfi fan switch has the same size/thread as the polo rad and the fwd thermostat so saves me buying a new one
So I reckon I'll leave it on the fwd thermostat to plug the hole there and leave the polo fan switch on the rad to plug that hole and then just route my expansion bottle bottom hose to a T joint on the bottom hose rather than attempt making something to fit into the polo rad fan switch hole.
fitted to polo rad
fitted fwd thermostat
