You don't have to move the plants to drain the tank. Leave the gravel and plants where they are and just syphon the water out and refill it. Or just let it run for a few days and see if the ammonia stops by itself.
We have the same water supply company as all 4 of the shops we've visited and none have mentioned it being an issue.
Just popped back to the shop where we got most of our set up.
He's going to run his own test on the rock so we'll speak to him in the week on that.
As for the tank. Job 1 is to double check some tap water with some plant food just to rule out any false Nitrogen based readings.
Then if that comes back negative we will be going for a fishless cycle using the decayed fish food method. Might as well try working with what we've got.
Thanks for all the help and information. Hopefully we can get the cycle going and all will be well!