While accelerating, the dashboard lit up with a check hybrid system message and the car lost all power. However, it was still able to 'coast' and I was able to pull into a layby and call out Toyota rescue. Long story short, after being recovered to MrT, the diagnosis is dead inverter. Cost to fix approx £1700 but should be covered by the toyota extended warranty. Might be done by the end of the week subject to parts and labour availability.