Jonny, bad news - if the mice want to get in, they will. We have tried natural (herbal) deterents, moth balls, even poison, those darned mice go through anything when they're of a mind. (We have a problem with them nesting in the Sta-Rite heaters and chewing through the wires when they get hungry or bored) Water tubes will help fill the gap so leaves, etc don't get into the pool over the winter, but mice seem to be able to 'flatten' themselves down to < 1/4" to get where they want to go.
I read about a 'possible' fix for your problem some years ago - duct tape and steel wool. Make a 'mat' of duct tape that will cover the area in ? and stick steel wool to the back side (where it's not adhering to the cover and deck/ wall) - if they try to chew through the tape, they get a mouthful of steel wool and stop trying, because they have a lethal case of indigestion. ( or , depending on how you feel about mice)