Truma Ultrastore - air locked on electric

We have recently purchased a 2003 Ace Award fitted with a Truma Ultrastore water tank/heater. The caravan is in excellent condition as it's hardly been used. On our first trip with it last week everything worked perfectly except the water heater when on electric operation. 

We followed the instructions and ensured all of the air was out of the system by opening the hot taps until water came through when first setting up on site. We then switched the Ultrastore to electrical operation (I had tried this at home and it worked OK). The next time we came to use hot water all that came out of the tap was air again and eventually cold water. We swtiched off the electric switch for 5 minutes (as directed in the manual) and tried again but the same thing kept happening.

We resorted to using the gas supply to heat up the water and this worked perfectly for the rest of our stay.

I'm thinking perhaps the thermostat has failed and it is boiling dry but there was no noise noticable from under the bunk and no smell. Of course I checked the winter emptying tap was closed as I'd already been caught out by that when I first filled it whilst testing at home.

Anyone any other ideas?