iMessage not recognising contacts that have iMessage

On my ipad 3, every time I add a contact who has imessage it appears red. I understand what the red colour means (that they are not registered with imessage) but I know for certain that they are as I can send iMessages to them from my iPhone 5 (blue messages).

This only started occurring with the iOS 6 update.

The only thing I've managed to do to get it to send is to add the country code (+44) and delete the zero and then it recognises it. Obviously I don't want to go through all my contacts and change every single number.

I also don't receive any iMessages on my ipad unless there is already a message string open from one where I've used the +44 workaround.

Any ideas? It's driving me mad!