Surprising no one's made an alloy Header tank for these, seems a pretty common fault 2003 4.4 Adriatic Blue FF

This happens to mine. When I give it the gun the check coolant level warning flashes up. Check the level when I get home and it is slightly above the line (engine still warm). When engine cold it is right on the line. I was concerned so purchased a sniffer kit. Used the kit and came up negative.
I guess I should drain the system and put in the correct ratio OAT coolant (50:50). Previous cars:
S-Max 2007-2013 (only diesel I've had, good car but expensive when diesely bits go wrong, so what's the point?)
Galaxy 2001-2007
Mondeo Estate 1997-2001
Sierra Estate 1993-1997
Uno Turbo 1987 -1993
Fiesta 1984 - 1987
Fiat 127 1982 - 1984