3 months ago I painted a south facing window-cill with Dulux Brilliant
white non-drip gloss. Now I notice that where there are items on the
window-cill the paint has discoloured underneath them...Has anyone got
any ideas as to why??

Solvent paints often change shade over time, particularly in response to UV
radiation from the sun. Consider using water based paints which generally
seem to suffer less from this.

Christian.