504 GatewayTime-out

(The server didn’t respond in time.)
I installed 4.2.2 on a multi-site (https://www.soutarwriters.co.uk) about 12 days ago. It seemed to work ok, and I have posted to the site since. I’m now getting the 504 message and can’t login. I’ve re-installed 4.2.2 components, but no joy. I reverted to 4.2.1 which had worked well, but it can’t get past the 504 message now either. I’ve tried disabling plugins, no joy. I contacted my host supplier who has checked out server problems (none) and can’t login either. (Plugins are: akismet, go-social, nextgen-gallery, pjw-mime-config, prezi-shortcode, si-contact-form, tinymce-advanced, twitter-widget-pro, user-role-editor (the most recent install), wordpress-seo, wpaudio-mp3-player.) Any ideas?