Hiya,
I just discussed it with Iminei (apologies I didn't get on it earlier, I'm currently on support for work too and we just had an issue!)
This isn't anything to worry about, our certificate expired. I'm not sure why it didn't automatically renew as it should have done, but either way there is no security issue with the site. I'm on support with our hosting provider now to get the cert reinstated and find out why it didn't automatically renew.