Ok so been doing some more reading (theres a lot of guides on here!)
I was under the impression that a hardness of 16 was classed as Hard Water (the book I got with the tank says 16.5 or above is 'vary hard water - not suitable for aquarium fish'!)
After having read around, 16 is more like 'Moderately Hard', which now doesn't really seem too bad, my bronze cory is apparently quite happy at this level, and there are plenty of other fish out there that will also do well.
So I guess my question is probably now not needed, its good to know about the rainwater, but I think it is just going to be far easier to leave the water as is, and find fish suitable for my water.
Just goes to show you can't trust anything you read!
Thanks