I'm afraid I don't know the answer to your question, but there is someone who might!
@mellielong
For years I had a similar conundrum in Cambridge, with hundreds of ladybirds that invaded my office every winter - should I eject them, when they were likely to die of cold, or should I keep them indoors, where there were no aphids for them to eat? I usually copped out and ejected some and kept others - since they always came back the following winter, one or other approach worked! I guess you can't do the same with your little caterpillar, barring cutting him in half... (joke!)
How did I eventually resolve my Cambridge dilemma? I moved to Oxford!
