Paul Easton handily posts the ratings information for London stations as a graph.
Have a look: http://pauleaston.blogspot.co.uk/
You might then want to alter your opinion of how much of a ratings hit this show is.
Andi does have the Saturday graveyard slot though. People who are into the music played on his show are probably out and can't listen to his show so it's hard to tell whether his ratings would increase or decrease if his show moved to an earlier time slot.