Thanks guys for the advice and support, it is much appreciated.
Yesterday, luckily a friend was round so we got to empty the tank, very sad to see so many dead shrimps, so all the plants etc are in a bucket with the old water. I fished out 10-20 shrimps that seemed ok (not got the courage to look yet today) and in the process of getting those into the old 15L temporary home for probably a week. I ordered the substrate 'shrimp king' and that was sent yesterday and should arrive today, and a thermostat to stick on the outside.
I will look at what you recommend JayC at some later stage when it is all back UP and RUNNING again, and I can relax a bit!
You are right JayC, although it is a disaster, it is good to KNOW what caused it and as you say heaters break, though before when they have broken to where they didn't work, which just meant the occupants just got a bit cold and caused no problems, but it is a valuable lesson learnt. I will get thermometers for the other tanks as well, but only ordered one at this stage to see if I like it and it works etc.
I have ordered 2 x 4L shrimp king soil as they say that needs no preparation and very quickly the shrimp can then go back. I was planning to put double the amount I used previously (so thats 6L, though that was a different substrate) so it will be about 1.5 inches. Any advice on that would be appreciated? And as you say JayC I will pretty much use the plants etc from before.
I appreciate the advice and thoughts a lot, thank you both!
edit - Total wipeout as the ones I fished out still died! It does mean I don't need to hurry to set everything up at least and I will need to go shrimp shopping sometime in the summer.
Simon