Here is an MSN Paint! The blue is where the cat comes in. The orange shows where the cat has gone to; and F is where it's eaten the food.
I realize I should try moving his food further and further away from the window, but he has to be able to see the food from the window to want to come in.
Also even when he went along the corridor, he was fast enough to be back in the lounge and out of the window by the time I had stood up...:
One idea I did have was to put the food in the conservatory, where he could see it from the patio. Then maybe he will come in through the lounge and walk all the way there, and I might have a chance to close the lounge window. The conservatory would then be the room we'd leave him in to get acclimatized, and it's where his litter-box cage is.