When I try and watch sky sports/movies through the to anywhere app on my iPhone the message "device limit reached" pops up, it's been working perfectly fine for a year and I haven't deregistered my device. Any help appreciated!
Many apologies that you're unable to watch Sky Sports on your iPhone due to device limited reached.
TV Anywhere and Sky Sports are both separate platforms and can use different devices for both.
If you sign into your TV Anywhere Account (on laptop/PC), on the right under My Profile you will have Device Settings and Sky Movies & Sky Sports. Through Device Settings this will show the devices registered for TV Anywhere and Sky Movies & Sky Sports will show devices for Sky.
Does it show your iPhone as registered if you select Sky Movies & Sky Sports?