After about 30 miles, noticed radiator level had dropped to the level of the core - topped up presuming just air from the re-build.
Yesterday went about 20 miles, and noticed coolant on the back wheel - header tank was full, and now overflowing.
Thought this could possible be rad cap not sealing properly, so cleaned it and gave it a run - same problem. Then added a thin rubber washer below rad cap to make sure it was properly sealed, run it again and the waterpump casing gasket blew

Again temperature didn't move while it was filling the header, or with the washer below the cap, and only shot up (as expected) when the gasket let go. So it's not running hot, but the system is over pressurising, so all I can think of is head gasket ??
Thermostat is new. I built the head onto the barrels before installing on the crank cases. Used a pattern gasket set, and a smear of Kawa-bond.
I fear the worst....................Any thoughts ?
Cheers,
Tommy