Motorhome Fresh Water Tank

I am new to motor caravanning and have a (2nd hand) Elddis Autoquest 180 (2008). When filling the fresh water tank, I've noticed that I have to fill it very slowly or the filling pipe just backs up with water and overflows back out the filling point (at first, I thought this was a sign the tank was full and ended up driving around thinking I had a very small tank!). 

Looking under the van, I can see the fresh water tank has an overflow / vent outlet on the opposite side from where the filling pipe enters the tank. Looking up this with a torch, it appears to be partially blocked with a piece of cork or similar. If the vent was blocked, this would explain why I could only fill the tank slowly, as not enough air would be escaping out the vent during filling.

If the cork is a blockage (and goodness knows how it got there), one solution will be to try and poke out the cork with a sharp knife. But before I do this, I'd be grateful if anyone with better knowledge of the workings of a motor caravan fresh water system can confirm that it's unlikely that the cork is meant to be there - for example as part of some valve or seal on the tank vent. The last thing I want to do is do some damage that causes other problems! 

Many thanks!