Peter Gillies post gives a very plausible reason for loss of control after a double engine failure. Since the AAIB report indicates that there wasn't an immediately obvious mechanical failure and that there appeared to be sufficient fuel in the tanks to return safely to base, what could cause the engines to fail.
My question is, are their any 'forbidden' attitudes when fuel levels are low, although no low fuel indicator lights are illuminated? For example if the helicopter adopted a steep nose up or nose down attitude, or banked sharply, would the fuel feed pipes be exposed to air above the fluid levels in the tank(s)?