Hi James,
I installed some new plug-ins recently and from what I read about previous posts on this it could be that. One of them was Yoast SEO which I'd really like to keep (I've found it really useful)
The high traffic + installing the new plug-ins has happened within the last 2 or 3 days. This error has also happened in the last 2 or 3 days so it could be either I guess.
The only other thing I've changed recently is adding a new sub-domain to this hosting account which doesn't seem to be affected by these issues. - Then again there's no way of knowing.