[NigelB]

Chris
Mine sounds similar to yours. A blanking cap on the radiator (with the outlet in the neck connected to the expansion /header tank) and a pressure cap on that. If the pressure cap on the expansion tank released coolant would flow to eeeeeeeeerrrrrrrŕr, Tarmac!!, There's more than one way of skinning a cat.......!!

In mcramsy's case, with radiator and expansion tank already installed, a pressure cap on the rad seems the simplest option. The only issue I see is how he will fill / top up the cooling system once the nose cone is on. There's not a lot of room in there to get to the top of the radiator.