Trickle charging a Hybrid via the CTEK is fine Ed. My GS lives outside too, but with the mini jump starter, it can start most cars batteries which are flat.
My Merc estate has covered just 192 miles in the last 12 months and it has started first time every time each week. I moved it forwards and backwards on the Drive yesterday, just to satisfy myself that Brakes etc were not solid. It has been in PARK with the Handbrake off.