I should also add the sub tank light is on, I've checked for fault codes but there are none. Disconnected the battery for an hour and reconnected it which fixed it but next morning it was there again.

I'm guessing the switch for the sub tank is faulty as it doesn't click when pressed just feels soft with no resistance and minimal travel and also doesn't light up. Is this normal or is the sub tank switch meant to click on and off and light up?