How do you get to the diaphram on macadee DVC0300 bottom inlet valve?

I have just installed a cyctern provided with a Macadee DVC0300 inlet valve. For the first few flushes it filled the cystern perfectly but since then it takes and hour to refill. I suspect some debris has got into the valve but cannot find how to get to the diaphram to clean it. Has anyone an idea and or an exploded diagram?