I have done, and it has since come back up. Odd, as there are no error logs that I can see.
I'm thinking my host is going through a bout of unreliability, as this is the second time the site has gone down in 24 hours, but only the first (I think) since we migrated to them about a year ago!
I'm awaiting their response, hoping they have an explanation as to what's going on!
Thanks for your advice :)