OK, here's the full lowdown on what's gone wrong.
1&1 upgraded the Linux server software and also upgraded the version of Apache Server while they were at it (they didn't tell us they were doing this though).
The result is that Apache is not parsing php files and that has broken the site. We have a support ticket raised with 1&1 as the issue is on their managed server side and we don't have access to fix it. We are in their hands and I am severely questioning their "24/7 support" claim...... :/
On a scale of not being happy of 1 to 10 (10 being super duper unhappy), I am about a 36 right now and counting.....