Turns out over 18 months ago the site was hosted with the same company and the zone file still exists and points here.
I'm furious. The site gets around 2-3,000 visitors a day and a lot of regular users who are going to be very confused. The hosting company don't appear to be too concerned about sorting it out. Great, at nearly £800 p/a for the reseller account I thought they might be a little more efficient!