So, fitted the new hose and tank today after flushing the system out.
Topped it up and added 2 litres of anti freeze.
Ran it up to temp, needle is nice and steady on the gauge, got the blowers in the car running nice and warm and both sides of the radiator at the bottom are hot.
All seems good but the water level is all over the place. I though id got the correct amount in the tank but after running it for a while, turned off and took the cap off the level has dropped significantly. But when it running it holds steady at the correct level.
Any ideas why?

Thanks.