Cosy is NOT a good tariff if you want to run you ASHP efficiently..
Basic principles of ASHP/GSHP are to run them “low and slow” basically at the lowest flow (water) temperature that is enough to heat your house.
Cosy works on the principle of ramping up your heat pump for a few allocated time slots that are slightly cheaper than SVT. it does not take into account that there will much higher peak rate paid by the customer for the time slots when cooker dinner etc.
Really not a good tariff, anyone who runs their ASHP using the most efficient settings will agree