Ford Cargo 0813?

I recently added a second battery to my vehicle (wired in parallel) to boost starting in cold weather. All seemed to work well and the vehicle turns over and starts really well. Driving the vehicle today (first proper journey since the work), the fuel gauge and the temperature gauge both slowly crept right up to the full/hot and went off the scale. At one stage duriong the journey the gauges came down slowly to the full mark on the fuel gauge and the red area on the temperature gauge but then went back off the scale again.

The fuel tank was not full and the vehicle was not overheating. All other gauges worked as normal (although air pressure and oil pressure are not electrical and the rev counter is electrical but feeds direct from the alternator).

The vehicle runs fine and all other electrics seem to work ok.

Does anybody have any ideas what the problem could be?