I have a new Octavia SEL mhev 1.5.

On two occasions I have unlocked the car with Kessy and failed to start the engine on pressing the start button. Isn’t it the case that as the system has recognised the key to unlock the car that it should also be able to start the car? 

 

Correct, but the sensor thing for ignition may not have sensed your key(different system to unlocking the car). Sorry I don't have the handbook to hand to screenshot where that sensor is, but I have a feeling it's near the centre console/gear stick/gear selector. Do you drive a right hand car, and was the key in your right hand pocket? Phone in there too? 

Yes I do drive a right hand car and key in right pocket! Thanks for this. I’ll move the key to the centre console and see if it solves the issue. 
 

Left hand pocket might be worth a shout, I always worry I'll leave my keys in the holder thing in the middle and be locked out the car! We're creatures of habit though aren't we (always had car keys in my left pocket)

I carry the key in my RH pocket as a rule and it works fine, however once or twice the 'key not detected' message has popped up, so that may be the reason. Shifting it to the LH pocket is a good idea, as is never placing it in the console!

IIRC, the sensor is under the cup holder.

 

I've had that before when I had the keys in the same pocket as an RFID wallet......