One of my radiators (bathroom - Twinfast valve - Worcester Highflow
Heatslave Boiler- 22 years old) makes a noise as though loads of air is
getting into it - especially after the boiler diverts back to C/H after a
bath / shower.

However, when I check the bleed valve there is never any air.

What is going on? It sounds like loads of air is entering at times.