Thanks again for all the great advice JayC.
I did the weekly maintenance of the FISH tank today and have kept the old water (sponges were rinsed in it so nice and dirty) while I mull everything over.
I checked the cupboards and have everything except an electrical adaptor - cheap and easy enough to get! Main thing now is whether I can re-arrange stuff to make room for the 15L tank near the new shrimp tank (probably wait until friends come round for that), as when I used it before it was on the dining table and that wasn't a good idea at all - always in the way etc, but it was only a TRIAL shrimp tank at that time?
I feel more confident it should be possible thanks to all your helpful advice! Thanks for taking the time to do this JayC.
Simon