iMessages issue on MacBook Air Sierra

Hi there, I'm hoping someone cal help me with the iMessages app on Mac Sierra.

I don't know if anyone else is having any issues with the iMessages app for their MacBook or iMac but I have two issues that seem to have appeared out of nowhere.

Firstly, I can't seem to message anyone on my contacts list using the app but I can receive messages and when I do the messages don't come up with contact names just their number starting with +44 which I've never noticed before

The other issue I'm having is that on the app when I go to write a message, it's coming up with old contacts that don't seem to be stored anywhere and have been deleted before I got this MacBook Air so I don't know where they're coming from.

I've only found this issue on my MacBook - my iPad and iPhone are absolutely fine. Hoping someone can help me fix both of these issues or let me know if there's a known bug! Thanks