haha ace! slippers :)
i've just got back now and (didn't even mention the laces thing) she's suggested the same answer, either the footwear is too tight around your toe or there's a lack of blood flow to the foot, which is typical on cross trainers / exercise bikes.
she suggested you try either doing shorter sessions on that particular machine, or use a different cardio exercise that means you're moving the muscles in your foot if it's making you suffer - as you'll be constricting your toes repeatedly, the bloods not getting the chance to flow back again, which is why it's numbing.
i have to admit, when i was complaining of my foot problem initially, she suggested different trainers (asics gt130 to be specific - on sale at M and M direct, mens and womens styles) and the pain is close to none existent now as long as i tie them right. my husband has fallen arches and swears by them too. i had reebok classics before (they were cheap!!!) and apparently they're terrible for the gym offering no support or cushioning whatsoever. so perhaps it could be simply down to the actual footwear?
obviously i'm not pretending to be a professional, but thats what worked for me!