2003 MG ZR Question to hopefully rule out head gasket

Engine Cooling problem
2003 MG ZR 4 cyl Front Wheel Drive Manual 67K miles

Hi

I Have a problem with my MG ZR.

I susupect head gasket, but i am not 100% sure.

i have milky residue inside the water tank, non under the oil cap.

there is no obvious sign of leakage from either the rad or any of the piping, and the temperature gauge shows normal 1.e halfway between hot and cold.

please can you give me some advice or possible causes