I'm gutted to hear this. So sorry, Simon.
Unless your home gets awfully cool, you might find that your shrimp tanks do just fine without a heater. I have 11 shrimp tanks and only one is heated (to 74F/23c because of one fish). They slow down breeding in winter, but I don't mind that. My tanks at my shop drop down to 66F (19C) in winter months and I've never had losses related to cool temps. On the other hand, I have lost hundreds upon hundreds to tanks getting too warm. Just food for thought and if your place's temperatures fluctuate a ton this might not be ideal especially in a smaller aquarium. But I once lost power at my shop in the dead of winter and my tanks got down to around 11c and and Crystal and Neo shrimp didn't die. They were motionless and not happy so I made sure it took days to warm back up by setting the room thermostat quite cool and inching up slowly, but that was last winter and I still have those colonies (as well as some fish in other tanks that went through it). I would not recommend such extremes obviously and realize I got very lucky.
Just a head's up you might already know about, but that Dennerle substrate will spike ammonia the first few weeks. It's probably my favorite out there, but there's quite the waiting period before it's safe to add shrimp.