Hi Andrew,
I have almost the same setup and issue:
- Freezer in a Separate Garage with a Fibaro FGFS 101 for monitoring
- Double Brick and Concrete House
I tried a bunch of extenders, mucked around with placement etc. I found most of the extenders were rubbish in this situation (eg ZW117)… but the standout was the Aeotec ZW098 Light Bulb. This thing looks like it has a fully exposed 1/4 Antenna and unlike power pack extenders you tend to locate them in more open areas. These and the best z-wave repeaters I’ve got - they really punch through brick walls! Next best is the older Aeotec ZW075 Power Pack, better than the newer models and all the small extenders (but it is a bit larger).
The other thing I did was moded my Z-Wave stick for better reception.
More info on all of this at 
Guide: Aeotec Z-Wave Stick - Adding an External Antenna