If I remember correctly, you still get illness benefit and it won't affect it. If you have income protection insurance, they'll deduct trr from their payments to you. By the way, your income protection will most likely say you're not covered no matter what. Request a review of their decision and they usually will agree then if you can get your gp to write something in your favour.
You need to fight for your rights unfortunately but I would highly suggest keeping a really detailed list of every time you have contacted HR, their response and response times. Further down the line you might find this will come in useful!