2013 Symbol winterisation - frost valve

We are in process of draining our van.  Water & waste tanks are empty, but we are struggling a bit to drain the water heater.  The manual says to turn the knob on top of the frost valve 180 degrees, and then shows a button that pops out.  On our valve, the button only pops out about 2-3mm compared to apparently much more as shown in the drawing; and it pops out after turning the knob 90 degrees. 
Is this correct operation?
The problem being, that no water drains out.  We don't see any dripping out under the van and it is still visible in the pipes around the heater which in our van are transparent with red/blue stripes for hot/cold.  I was assuming that if the sink tap is open, then the water heater will simply drain once the frost valve is operated.  Is there some other trick?
Any other winterisation tips?

I would imagine water from the heater would go into the waste tank,perhaps someone can clarify that.I usually leave the drain taps open, including the hot water heater tank and go for a drive, this clears the water from the pipes and filters. This should also clear the last of the water in your tanks.

I cannot remember how far the frost valve button comes out on our Lancashire but water definitely drops out underneath the van. Did you open the taps and turn them to the midway position between hot and cold?

Yes, the taps are half way.  I guess I could look underneath tomorrow and see if the drain is visible.

Water should drain out underneath the van. Got my van ready for a French journey today, and filled with water. Thought the frost valve was closed, but drained out . The button does only come out slightly. Reset and all was well. Seems that when I put the pump on, the water drained out very fast. Maybe put the pump on and try.

Now I am puzzled.  Checked first that the drain outlet (as above, dumps straight onto the ground) looked clear and it was.  Started  to fill the tank and water immediately came out of the frost/dump valve.  The valve was indicating open, (how I left it yesterday) and once I turned the water fill off,  the pipes looked empty. (yesterday the pipes were clearly full with a couple of air bubbles visible.  Today obviously empty with a drops of water visible here and there)  And I assume the heater is empty too.  The valve would not close - still too cold?
Last night was very cold here, so I can only assume that the frost valve opened and dumped the water as it should BUT the valve would not operate manually when I tried it yesterday.  I'll investigate again on a warmer day but for now I think all is drained.
Thanks for the support.

What did you mean by "slightly" for the button? If I remember well my button will protrude about 1cm or so. Did you notice any difference in the button today? Did you turn the knob on top enough previously?

On our valve, the button only pops out about 2-3mm compared to what looks like the 1cm or so (as you describe) that is shown in the manual; see above "Tengah74" seems to say the same.  And no difference today.  The knob on top turns right round - and the button pops out after turning the knob 90 degrees, i.e. right angle to water flow direction.  The button will not latch back in today - it's quite cold here.  Our van was first registered (not to us) in Jan 2013, so maybe similar in age to yours?