[the lodger]

To finish this thread I managed to buy the part from the Netherlands.

The only way I found to fit it was to take off the complete filler cap assembly which allows better access to the hole the rubber tube fits in, and then it's easy to fit.

I think the tube acts as a drain for the key hole in the cap ( and has nothing to do with venting ). That makes sense to me otherwise water could get in when the key is fitted and it would otherwise drip directly into the tank.