Leak on Fresh Water Tank/ SYMBOL

Please can anyone help to identify the problem.     Just succesfully filled up Fresh Water tank after 'Wintering'.  Have added sterilizing solution to pump through pipes to disinfect (will flush later).
 
When I turned on the pump to get water succesfully through to all taps, I heard water leaking from under the Van - after checking the location - found water leaking from the large black (2" ish) ribbed pipe that comes out of the fresh water tank to supply something. Sorry Lady on her own with no technical experience.  When I turn the pump off - the leak stops.    Can anyone send me in the right direction to what to do next or who needs to fix it.  Thankyou for your help.    (the yellow switch in the water heater compartment HAS been moved back to the horizontal position after wintering).

Hmmm... Well the 2" ribbed pipe is usually the ~inlet~ pipe, that connects from the filling point to the tank. It's hard to see why that should only leak while you have the pump on! Though I guess it might have been coincidence that you happened to turn off the pump just when the water level dropped below where the inlet pipe is leaking?

TBH these types of leak can be very hard to track down - water may be dripping off a pipe, but the point of origin could be something completely different, and it's just that the water is dripping onto that pipe higher up.

Possibly a good starting point would be to expose the pump itself and check that when it's running you aren't getting any leaks from the connectors or the filter assembly - which can sometimes crack during winter storage.

Hi

Last year when I shut the Yellow Valve to the off position it was leaking underneath running down the fresh water tank. The Yellow Valve has two closed positions which can be used to shut the draining down pipe. In my case the lever was fouling a water pipe and did'nt go all the way to close position. So changed to other way ok then. Hope that helps.

I just replied to this but it seems to have disappeared somewhere so here is a short version:
The Truma Frost Control valve is open.
Find the Frost Control unit.
Make sure the Rotary Switch is in the Operation postion in line with the water pipes.
Press the Drain Valve push button IN. Click.
Pete

   THANK YOU  THANKYOU  to all of you who have helped me find the problem.   This is my first winter and so drained down the water heater AND EVERYTHING ELSE. (and now have to reverse the process). 
Firstly mojo - yes it was another pipe above that was dripping down onto the black ribbed pipe.  I laid down and crawled right under the van with my toch so I could see it was the heater drainage pipe where the water was coming from.  Then Stewart said the yellow flick switch can be a little vunerable with its position  AND THAT's the one that did the trick.  I tried it on the opposite side and wow!!  As it's the first time doing all this, I took for granted that it would all go straight forward.  Now that my first winter is over - I'll know for next time.  I have said it before and I'll say it again.  THIS FORUM IS GREAT for others help.  THANKYOU EVERONE.  I'm off to the Cotswolds on Monday with no worries.