OK so my little buzzbomb has just developed a little issue with start stop function. When I stop the system tries ro cut the engine but does not do it fully. Engine is still running but only just, car shudders. When its like this the car does not do anything when you depress pedals. Only way to restore the car is to turn it off and restart it

I think it's the engines way of telling you that Start/Start is just a gimmick and it's had enough !!

Turn off Stop/Start

This does sound a bit odd though. In my experience S/S either works or (usually!) not, either because of low battery charge or some other fault (e.g.I once had an internal short in the starter motor). Never experienced a "partial shut down". 

Might help to know if it does this when coming to a stop if the S/S is disabled with the switch, as suggested. 

Likewise has the car been regularly serviced, etc? In view of ongoing debate into effects of debris from cambelt deterioration, was this changed when due last year?

 

Sounds like engine is being requested to stop, but running on for some reason.  Does it ever happen when you manually switch the engine off, or only when the stop/start cuts the engine?