We also have our bottle on the inside of the cage (we have a C and C) We use two metal spring bottle holders to keep it in place and stop it slipping. You can get the springs from eBay.I actually have 2 bottles per hutch at different heights as my lot drink in all different ways. Also, I find that the classic bottles can be a little short on the spout, so I hook them inside the hutch and use spare holders around the neck of the bottle to hook into place, if that make safe. It's more difficult when changing the bottle but worth it in my mind. You can get the metal neck holders supplied with the glass bottles which work really well.
Don't worry it's common for them not to drink much.Thank you both. It seems like they are drinking it. I have been marking the bottle. But it's not a lot I don't think. I will have a look online.