I work in Housing - please pay the full rent as per your contract.
And put in a complaint about not receiving the discount on your water bill that you are entitled to (the HA pays less water rates because you are on a low income so they should reduce your water charges due to this). The HA have 10 days from when you send them your complaint to respond. Put your complaint in via email so you can track the replies etc.
If you are not happy with their response, ask for your complaint to go to stage 2 of their complaints process. They have 20 days to review their decision and get back to you with their response.
If you are still not happy with their answer, take your complaint to the Housing Ombudsman and provide the HAs stage 1 & stage 2 responses at the same time. They will not consider your complaint until you have gone through your own HAs complaint process.
You can have a look at the Ombudsmans rulings online to see if they have already had similar complaints as that will tell you what the likely outcome will be.
I would also dig out your last years rent notification letter which, by law, has to break down all the charges your HA is charging you for - make sure that you check the water charge part to make sure that the figure on the rental charge letter is the correct figure.
Id be really interested to find out what happens in your case - please keep us updated.