I had a similar problem a week ago using a registered device which has previously worked fine. No device changes left, so I had to ring up and ask them to reset the device count so I could re-register my phone.
Many apologies that you're unable to watch Sky Sports through your iPad due to device limit reached.
When I checked your account, there is an available device change for you. If you sign into your TV Anywhere account, on the right hand side under My Profile select Sky Movies & Sky Sports from a laptop/PC. You can remove the iPad from the list, then go back onto the iPad device itself, go into Sky Sports and start streaming a channel to prompt for re-registration.
Hopefully this will work for you. Let me know how you get on